In the past, the metric `agent_bootstrap_seconds` with label
`scope=restore` was used to publish some endpoint restoration
related metrics.

With the modularization of the legacy daemon init logic, some of the
endpoint restoration logic no longer reported its duration to that metric.

In addition to this, the label `scope=restore` was pretty generic, because
other restoration logic was using the same label (e.g. identity restoration).
And the metric didn't take into account that the actual regeneration of
the restored endpoints was executed asynchronously.

To shed some more light into the endpoint restoration process, this
commit introduces two endpoint restoration specific metrics.

* `endpoint_restoration_endpoints` - with label `phase` & `outcome`
* `endpoint_restoration_duration_seconds` - with label `phase`

The following phases of the endpoint restoration process report the metric.

* `read_from_disk`: Reads old endpoints from state dir
* `restoration`: Restore old endpoints. includes validation & IP re-allocation
* `prepare_regeneration`: Trigger asynchronous regeneration
* `initial_policy_computation`: Duration until the initial policy for all restored endpoints is computed
* `regeneration`: Duration until all restored endpoints are regenerated

e.g.

```
root@kind-control-plane:/home/cilium# cilium-dbg shell metrics endpoint_restoration
Metric                                         Labels                                                Value
cilium_endpoint_restoration_duration_seconds   phase=initial_policy_computation                      0.006262
cilium_endpoint_restoration_duration_seconds   phase=prepare_regeneration                            2.506187
cilium_endpoint_restoration_duration_seconds   phase=read_from_disk                                  0.002105
cilium_endpoint_restoration_duration_seconds   phase=regeneration                                    2.270423
cilium_endpoint_restoration_duration_seconds   phase=restoration                                     0.101983
cilium_endpoint_restoration_endpoints          outcome=failed phase=read_from_disk                   0.000000
cilium_endpoint_restoration_endpoints          outcome=failed phase=regeneration                     0.000000
cilium_endpoint_restoration_endpoints          outcome=failed phase=restoration                      1.000000
cilium_endpoint_restoration_endpoints          outcome=skipped phase=restoration                     1.000000
cilium_endpoint_restoration_endpoints          outcome=successful phase=initial_policy_computation   4.000000
cilium_endpoint_restoration_endpoints          outcome=successful phase=prepare_regeneration         4.000000
cilium_endpoint_restoration_endpoints          outcome=successful phase=read_from_disk               6.000000
cilium_endpoint_restoration_endpoints          outcome=successful phase=regeneration                 4.000000
cilium_endpoint_restoration_endpoints          outcome=successful phase=restoration                  4.000000
cilium_endpoint_restoration_endpoints          outcome=total phase=initial_policy_computation        4.000000
cilium_endpoint_restoration_endpoints          outcome=total phase=prepare_regeneration              4.000000
cilium_endpoint_restoration_endpoints          outcome=total phase=read_from_disk                    6.000000
cilium_endpoint_restoration_endpoints          outcome=total phase=regeneration                      4.000000
cilium_endpoint_restoration_endpoints          outcome=total phase=restoration                       6.000000
```

Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>