motivation: align CoC across the swift project

changes: change CoC to link to the swift.org version

…tKit

Migrate CI to use GitHub Actions.

Motivation:

To migrate to GitHub actions and centralised infrastructure.

Modifications:

Changes of note:
* Increase minimum Swift version to 5.9
* Adopt swift-format using rules from SwiftNIO
* Remove scripts and docker files which are no longer needed

Result:

Feature parity with old CI.

### Motivation:

Catch potential data races at build time.

### Modifications:

- Enabled strict concurrency complete checking.
- Created a thread-safe wrapper `SystemMetricsProviderContainer` around
`SystemMetricsProvider` to synchronize access to the current provider.
- Changed `CPUUsageCalculator` to be a class, again the previous
implementation used a global variable that held a struct, and called a
mutating function on it.

### Result:

Fewer potential data races can sneak in.

### Test Plan

Ran tests locally, did not see any more warnings or errors.

Enable MemberImportVisibility check on all targets. Use a standard
string header and footer to bracket the new block for ease of updating
in the future with scripts.

Update the release.yml file with the latest label changes

CI use 6.1 nightlies now that Swift development is happening in the 6.1
branch

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Only apply standard swift settings on valid targets. The current check
ignores plugins but that is not comprehensive enough.

### Motivation

This package provides some system metrics on Linux derived from procfs.
These include CPU and memory metrics. The values returned from
`/proc/self/stat` need to be combined with system values. For example,
some values are returned in clock ticks or memory pages, so need to be
divided appropriately according to the system configuration for clock
ticks per second or page size, respectively.

Looking at the code, some attempt was made to do this: `_SC_CLK_TCK` and
`_SC_PAGESIZE` were used as divisors.

However, these values are the values of the _option_ parameter used in
the call to `sysconf(3)`, not the results themselves.

To illustrate this, here's a Swift program that prints the option name,
option value, and the result of `sysconf(optionValue)`:

```swift
#if !os(Linux)
import Foundation
print("error: This tool is only supported on Linux")
exit(1)
#else
import Glibc

for (optionName, optionValue) in [
    ("_SC_ARG_MAX", _SC_ARG_MAX),
    ("_SC_CHILD_MAX", _SC_CHILD_MAX),
    ("_SC_CLK_TCK", _SC_CLK_TCK),
    ("_SC_NGROUPS_MAX", _SC_NGROUPS_MAX),
    ("_SC_OPEN_MAX", _SC_OPEN_MAX),
    ("_SC_STREAM_MAX", _SC_STREAM_MAX),
    ("_SC_TZNAME_MAX", _SC_TZNAME_MAX),
    ("_SC_JOB_CONTROL", _SC_JOB_CONTROL),
    ("_SC_SAVED_IDS", _SC_SAVED_IDS),
    ("_SC_REALTIME_SIGNALS", _SC_REALTIME_SIGNALS),
    ("_SC_PRIORITY_SCHEDULING", _SC_PRIORITY_SCHEDULING),
    ("_SC_TIMERS", _SC_TIMERS),
    ("_SC_ASYNCHRONOUS_IO", _SC_ASYNCHRONOUS_IO),
    ("_SC_PRIORITIZED_IO", _SC_PRIORITIZED_IO),
    ("_SC_SYNCHRONIZED_IO", _SC_SYNCHRONIZED_IO),
    ("_SC_FSYNC", _SC_FSYNC),
    ("_SC_MAPPED_FILES", _SC_MAPPED_FILES),
    ("_SC_MEMLOCK", _SC_MEMLOCK),
    ("_SC_MEMLOCK_RANGE", _SC_MEMLOCK_RANGE),
    ("_SC_MEMORY_PROTECTION", _SC_MEMORY_PROTECTION),
    ("_SC_MESSAGE_PASSING", _SC_MESSAGE_PASSING),
    ("_SC_SEMAPHORES", _SC_SEMAPHORES),
    ("_SC_SHARED_MEMORY_OBJECTS", _SC_SHARED_MEMORY_OBJECTS),
    ("_SC_AIO_LISTIO_MAX", _SC_AIO_LISTIO_MAX),
    ("_SC_AIO_MAX", _SC_AIO_MAX),
    ("_SC_AIO_PRIO_DELTA_MAX", _SC_AIO_PRIO_DELTA_MAX),
    ("_SC_DELAYTIMER_MAX", _SC_DELAYTIMER_MAX),
    ("_SC_MQ_OPEN_MAX", _SC_MQ_OPEN_MAX),
    ("_SC_MQ_PRIO_MAX", _SC_MQ_PRIO_MAX),
    ("_SC_VERSION", _SC_VERSION),
    ("_SC_PAGESIZE", _SC_PAGESIZE),
    ("_SC_RTSIG_MAX", _SC_RTSIG_MAX),
] {
    let actualValue = sysconf(Int32(optionValue))
    print("Option name: \(optionName), Option value: \(optionValue), Actual value: \(actualValue)")
}
#endif
```


```console
% swift sysconf-test/sysconf-test.swift 
Option name: _SC_ARG_MAX, Option value: 0, Actual value: 2097152
Option name: _SC_CHILD_MAX, Option value: 1, Actual value: 63907
Option name: _SC_CLK_TCK, Option value: 2, Actual value: 100
Option name: _SC_NGROUPS_MAX, Option value: 3, Actual value: 65536
Option name: _SC_OPEN_MAX, Option value: 4, Actual value: 65536
Option name: _SC_STREAM_MAX, Option value: 5, Actual value: 16
Option name: _SC_TZNAME_MAX, Option value: 6, Actual value: -1
Option name: _SC_JOB_CONTROL, Option value: 7, Actual value: 1
Option name: _SC_SAVED_IDS, Option value: 8, Actual value: 1
Option name: _SC_REALTIME_SIGNALS, Option value: 9, Actual value: 200809
Option name: _SC_PRIORITY_SCHEDULING, Option value: 10, Actual value: 200809
Option name: _SC_TIMERS, Option value: 11, Actual value: 200809
Option name: _SC_ASYNCHRONOUS_IO, Option value: 12, Actual value: 200809
Option name: _SC_PRIORITIZED_IO, Option value: 13, Actual value: 200809
Option name: _SC_SYNCHRONIZED_IO, Option value: 14, Actual value: 200809
Option name: _SC_FSYNC, Option value: 15, Actual value: 200809
Option name: _SC_MAPPED_FILES, Option value: 16, Actual value: 200809
Option name: _SC_MEMLOCK, Option value: 17, Actual value: 200809
Option name: _SC_MEMLOCK_RANGE, Option value: 18, Actual value: 200809
Option name: _SC_MEMORY_PROTECTION, Option value: 19, Actual value: 200809
Option name: _SC_MESSAGE_PASSING, Option value: 20, Actual value: 200809
Option name: _SC_SEMAPHORES, Option value: 21, Actual value: 200809
Option name: _SC_SHARED_MEMORY_OBJECTS, Option value: 22, Actual value: 200809
Option name: _SC_AIO_LISTIO_MAX, Option value: 23, Actual value: -1
Option name: _SC_AIO_MAX, Option value: 24, Actual value: -1
Option name: _SC_AIO_PRIO_DELTA_MAX, Option value: 25, Actual value: 20
Option name: _SC_DELAYTIMER_MAX, Option value: 26, Actual value: 2147483647
Option name: _SC_MQ_OPEN_MAX, Option value: 27, Actual value: -1
Option name: _SC_MQ_PRIO_MAX, Option value: 28, Actual value: 32768
Option name: _SC_VERSION, Option value: 29, Actual value: 200809
Option name: _SC_PAGESIZE, Option value: 30, Actual value: 4096
Option name: _SC_RTSIG_MAX, Option value: 31, Actual value: 32
```

The results for our metrics were off because we were always using 30 for
page size and 2 for clock ticks.

### Modifications

- Remove the LinuxMain.swift: test discovery is supported on Linux for
all currently supported Swift versions.
- Update the internal documentation regarding the use of
`proc_pid_stat(5)`.
- Use correct value for CPU metrics, obtained from
`sysconf(_SC_CLK_TCK)`, not just `_SC_CLK_TCK`.
- Use correct value for memory metrics, obtained from
`sysconf(_SC_PAGESIZE)`, not just `_SC_PAGESIZE`.
- Write tests for both CPU seconds and RSS memory.

### Result

Fixes #40.