[cmd/opampsupervisor] Add a healthcheck endpoint

[douglascamata]

Component(s)

cmd/opampsupervisor

Is your feature request related to a problem? Please describe.

Currently the Supervisor doesn't expose any healthcheck endpoint that orchestration systems can check to know whether it's healthy.

Describe the solution you'd like

I would like to have a healthcheck endpoint in the Supervisor. I was thinking that we could verify that:

• The internal OpAMP server is running.
• Instance ID is correctly persisted into disk.
• Initial merged config is readable.

This endpoint shouldn't care at all about the health of the agent processes(es) managed by it.

Describe alternatives you've considered

No response

Additional context

Not sure if this idea makes sense, so please let me know what you think.

[github-actions]

Pinging code owners:

• cmd/opampsupervisor: @evan-bradley @atoulme @tigrannajaryan

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