## What does this PR do?

Adds support for zswap tracking:
1. From `/proc/vmstat`
2. From `/proc/meminfo`
3. From `/sys/kernel/debug/zswap/`
4. From `/sys/fs/cgroup/<cgroup name>/memory.stat`

The testing here is not trivial because:
1. zswap might be completely disabled at compile time:
https://github.com/torvalds/linux/blob/1f97d9dcf53649c41c33227b345a36902cbb08ad/mm/memcontrol.c#L1346-L1348
2. debugfs (`/sys/kernel/debug/`) might not be mounted at all
3. Access to `/sys/kernel/debug/` requires root

To deal with this situation, `metric/memory/memory_integration_test.go`
decides on the expected result based on `BUILDKITE_STEP_KEY`. This makes
it slightly harder to add new images to our pipeline (the
`ciExpectations` map needs to be updated) but ensures we are actually
reading zswap debug metrics somewhere in our tests.

Reviewer note: Please check my nesting of the `debug` metrics, it could
be that we prefer them at the same level.

## Why is it important?

Adds important kernel metrics of the zswap compressed swap backend. See
elastic/beats#47605 for more info.

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] ~~I have added an entry in `CHANGELOG.md`~~

## Testing Instructions

### 1. Local integration tests
All tests should run and succeed locally by default, if you want to
check for actual access of all metrics, use:
```shell
$ go test -c ./metric/memory -o memory.test
$ sudo BUILDKITE_STEP_KEY=manual PRIVILEGED=1 ./memory.test -test.run TestMemoryFromContainer -test.v
=== RUN   TestMemoryFromContainer
    memory_integration_test.go:64: Total: 33323646976, Free: 4660809728, Used: 28662837248
    memory_integration_test.go:70: Zswap exists: true, Debug exists: true (BUILDKITE_STEP_KEY="manual")
    memory_integration_test.go:72: Zswap: Compressed=557056 bytes, Uncompressed=217088 bytes
    memory_integration_test.go:72: Zswap debug: StoredPages=53, PoolTotalSize=557056
--- PASS: TestMemoryFromContainer (0.00s)
PASS
$ sudo setcap cap_dac_read_search=+ep memory.test
$ BUILDKITE_STEP_KEY=manual PRIVILEGED=1 ./memory.test -test.run TestMemoryFromContainer -test.v
=== RUN   TestMemoryFromContainer
    memory_integration_test.go:67: Total: 33323646976, Free: 19951370240, Used: 13372276736
    memory_integration_test.go:73: Zswap exists: true, Debug exists: true (BUILDKITE_STEP_KEY="manual")
    memory_integration_test.go:75: Zswap: Compressed=430080 bytes, Uncompressed=106496 bytes
    memory_integration_test.go:75: Zswap debug: StoredPages=26, PoolTotalSize=430080
--- PASS: TestMemoryFromContainer (0.00s)
PASS
```

### 2. Metricbeat -- memory metricset
```yaml
metricbeat.modules:
  - module: system
    period: 1s
    metricsets:
      - memory

output.console:
  pretty: true
```

Test this config:
- With old elastic-agent-system-metrics
- Without any extra permissions
- With `sudo`
- With `sudo setcap cap_dac_read_search=+ep ./metricbeat`
- After `sudo umount debugfs` -> debug stats disabled

<details>
  <summary>Before</summary>

```json
{
  "@metadata": {
    "beat": "metricbeat",
    "type": "_doc",
    "version": "9.4.0"
  },
  "@timestamp": "2026-01-28T09:43:47.196Z",
  "agent": {
    "ephemeral_id": "1242f8a9-2768-4582-82e2-f1a605d4f182",
    "id": "8b35a267-5085-44a8-a82e-e43a4af9af11",
    "name": "laptop",
    "type": "metricbeat",
    "version": "9.4.0"
  },
  "ecs": {
    "version": "8.0.0"
  },
  "event": {
    "dataset": "system.memory",
    "duration": 127644,
    "module": "system"
  },
  "host": {
    "name": "laptop"
  },
  "metricset": {
    "name": "memory",
    "period": 1000
  },
  "service": {
    "type": "system"
  },
  "system": {
    "memory": {
      "actual": {
        "free": 18093944832,
        "used": {
          "bytes": 15229702144,
          "pct": 0.457
        }
      },
      "cached": 14415081472,
      "free": 6065573888,
      "swap": {
        "free": 20558376960,
        "total": 21474832384,
        "used": {
          "bytes": 916455424,
          "pct": 0.0427
        }
      },
      "total": 33323646976,
      "used": {
        "bytes": 27258073088,
        "pct": 0.818
      }
    }
  }
}
```
</details>

<details>
  <summary>Unprivileged output</summary>

```json
{
  "@metadata": {
    "beat": "metricbeat",
    "type": "_doc",
    "version": "9.4.0"
  },
  "@timestamp": "2026-01-28T09:41:08.897Z",
  "agent": {
    "ephemeral_id": "9aa025fc-2b19-4e40-82d5-7603b3da8b99",
    "id": "8b35a267-5085-44a8-a82e-e43a4af9af11",
    "name": "laptop",
    "type": "metricbeat",
    "version": "9.4.0"
  },
  "ecs": {
    "version": "8.0.0"
  },
  "event": {
    "dataset": "system.memory",
    "duration": 127572,
    "module": "system"
  },
  "host": {
    "name": "laptop"
  },
  "metricset": {
    "name": "memory",
    "period": 1000
  },
  "service": {
    "type": "system"
  },
  "system": {
    "memory": {
      "actual": {
        "free": 17940357120,
        "used": {
          "bytes": 15383289856,
          "pct": 0.4616
        }
      },
      "cached": 15113895936,
      "free": 5096550400,
      "swap": {
        "free": 20952903680,
        "total": 21474832384,
        "used": {
          "bytes": 521928704,
          "pct": 0.0243
        }
      },
      "total": 33323646976,
      "used": {
        "bytes": 28227096576,
        "pct": 0.8471
      },
      "zswap": {
        "compressed": 112336896,
        "uncompressed": 464326656
      }
    }
  }
}
```
</details>

<details>
  <summary>Privileged output</summary>

```json
{
  "@metadata": {
    "beat": "metricbeat",
    "type": "_doc",
    "version": "9.4.0"
  },
  "@timestamp": "2026-01-28T09:48:09.372Z",
  "agent": {
    "ephemeral_id": "55e06c15-f7e0-44bf-b361-28e69015ec34",
    "id": "8b35a267-5085-44a8-a82e-e43a4af9af11",
    "name": "laptop",
    "type": "metricbeat",
    "version": "9.4.0"
  },
  "ecs": {
    "version": "8.0.0"
  },
  "event": {
    "dataset": "system.memory",
    "duration": 181346,
    "module": "system"
  },
  "host": {
    "name": "laptop"
  },
  "metricset": {
    "name": "memory",
    "period": 1000
  },
  "service": {
    "type": "system"
  },
  "system": {
    "memory": {
      "actual": {
        "free": 18898096128,
        "used": {
          "bytes": 14425550848,
          "pct": 0.4329
        }
      },
      "cached": 13020823552,
      "free": 7782682624,
      "swap": {
        "free": 20104949760,
        "total": 21474832384,
        "used": {
          "bytes": 1369882624,
          "pct": 0.0638
        }
      },
      "total": 33323646976,
      "used": {
        "bytes": 25540964352,
        "pct": 0.7665
      },
      "zswap": {
        "compressed": 312053760,
        "debug": {
          "pool_limit_hit": 0,
          "pool_total_size": 312053760,
          "reject_alloc_fail": 0,
          "reject_compress_fail": 8665,
          "reject_compress_poor": 0,
          "reject_kmemcache_fail": 0,
          "reject_reclaim_fail": 0,
          "stored_pages": 304752,
          "written_back_pages": 0
        },
        "uncompressed": 1248264192
      }
    }
  }
}
```
</details>

### 3. Metricbeat -- processes metricset
Add:
```yaml
      - process
    processes: ['.*']
    process.cgroups.enabled: true
```
to get zswap stats for the process metricset, check output with:
```shell
$ ./metricbeat -c metricbeat.yaml | jq '(.system.process.cgroup.memory.stats // {}) | with_entries(select(.key | IN("zswap","zswapped","zswpin","zswpout","zswpwb"))) | select(length > 0)' 
...
{
  "zswpout": 412,
  "zswpwb": 0,
  "zswapped": {
    "bytes": 1249280
  },
  "zswap": {
    "bytes": 283113
  },
  "zswpin": 107
}
...
```

## Related issues

- Relates elastic/beats#47605

…286)

## What does this PR do?

We don't keep a changelog in this repo,
[releases](github.com/elastic/elastic-agent-system-metrics/releases)
automatically generate a changelog from commits and the beats repo
imports this module and is responsible for the changelog.

## Why is it important?

Contributor clarity

## Checklist

- [ ] My code follows the style guidelines of this project
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added tests that prove my fix is effective or that my
feature works

## What does this PR do?

Remove per-test context timeouts and rely on the go test -timeout flag
(20m in CI). Bump sleep durations to 1200s to outlast the test run.

## Why is it important?

The container system tests used a shared 5-minute context.WithTimeout
for the entire permission matrix. On slower CI workers the 8-case matrix
in TestProcessAllSettings recently started exceeded this, causing
"context deadline exceeded" failures:
https://buildkite.com/elastic/elastic-agent-system-metrics/builds/929

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] ~~I have added an entry in `CHANGELOG.md`~~

## Related issues

CI failed in main and elastic/beats#48834

## What does this PR do?
Add the `modernize` golangci-lint analyzer (with omitzero disabled for
now) and fix all findings: strings.Split → SplitSeq iterators,
interface{} → any, manual min/contains → builtins and slices package,
reflect.TypeOf().Elem() → TypeFor[T](). Also fix a prealloc finding.

Consolidate the two golangci-lint CI jobs into a single matrix job and
bump golangci-lint to v2.10.1. This switches to running golangci-lint
fully on _all_ files which should be okay given the small-ish size of
this repo.

## Why is it important?

Introduces optimizations, consolidates Go syntax into new form, catches
omitzero bug.

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] ~~I have added tests that prove my fix is effective or that my
feature works~~

## What does this PR do?
encoding/json's omitempty is silently ignored on struct fields —
zero-valued structs are always serialized regardless of the tag. Go
1.24's omitzero fixes this by checking IsZero() or the zero value.

## Why is it important?

No downstream production code is affected (consumers use struct tags via
go-structform, not json tags), but this corrects a latent bug if these
types are ever json.Marshal'd directly.

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have added tests that prove my fix is effective or that my
feature works

## Related issues

- Blocks
#281

## What does this PR do?
Add support for collecting CPU bandwidth control settings from cgroupv2:
- cpu.max (quota and period in microseconds)
- cpu.weight (relative weight, replaces shares from v1)

## Why is it important?
This brings cgroupv2 to feature parity with cgroupv1 for CPU limit
metrics.

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] ~~I have added an entry in `CHANGELOG.md`~~

## Related issues
- Relates elastic/beats#47708