Emit metrics for sidecars

## What?

It would be useful to have sidecar info (e.g. status) emitted as metrics. Ideally the sidecar names/IDs and sidecar version would be included, but at the moment I'm not quite sure what that'd look like-- having the status would be the most important piece.

For example:
```
gl_sidecar_total{status="running"} 101
gl_sidecar_total{status="failing"} 2
gl_sidecar_total{status="unknown"} 1
```

## Why?

We'd like to be able to track and visualize sidecar state transitions over time (Prometheus + Grafana), and alert on certain situations (e.g. are there failing sidecars, are there an abnormal number of sidecars in the "unknown" state, etc.).

Related to #7944 (and maybe others).

## Your Environment

* Graylog Version: `6.1.0`