[cmd/opampsupervisor] Allow setting healthcheck host

[TylerHelmuth]

Description

Currently the opampsupervisor hardcode's the healthcheck extension's host to be localhost. This makes the extension unusable on kubernetes. This PR adds a new agent config setting to allow setting the health check host so that on k8s you can set the host to something like 0.0.0.0 or the pod IP.

Testing

Tested locally in kind using a local opampsupervisor image.

[codeboten]

Please add a changelog and update the readme as well to include the new option

[TylerHelmuth]

@codeboten changelog added, but where should I update the README? Currently the README doesn't document the agent's health check configuration.

[evan-bradley]

I think we should consider a different approach for this. This property predates the OpAMP extension offering meaningful health reporting information, and now that it does, we no longer need the health check extension to determine the Collector is healthy from within the Supervisor. As a result, I think it would probably make more sense to not have the Supervisor configure a health check extension, but but instead to suggest the user directly configures one themselves, either through remote or local config.

[evan-bradley]

I think we should probably just remove this property entirely, but if we keep it, can we make it an endpoint instead of just a port?

[evan-bradley]

where should I update the README? Currently the README doesn't document the agent's health check configuration.

There is a README in the Supervisor specification folder that we keep up-to-date. If we make a change here, we should update that as well.

That said, I think we should likely look at moving that section to the Supervisor's main README file.

[TylerHelmuth]

I think it would probably make more sense to not have the Supervisor configure a health check extension, but but instead to suggest the user directly configures one themselves, either through remote or local config.

I can submit a different PR to implement this strategy if you want.

[TylerHelmuth]

@evan-bradley what is the alternative strategy the opampsupervisor is already using to set health if not the health check extension?

[evan-bradley]

The Supervisor receives health updates in a format that should be functionally-equivalent to what's provided by the health check extension here: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/cmd/opampsupervisor/supervisor/supervisor.go#L796.

I think we should switch to using these messages to determine the Collector's health status instead of the health check extension.

[TylerHelmuth]

Ok I'll look into this switch.