[cmd/opampsupervisor] Document how to configure Supervisor's internal telemetry

[johannaojeling]

Component(s)

cmd/opampsupervisor

Describe the issue you're reporting

User documentation is currently missing for how to configure the Supervisor's internal telemetry. It would be useful to have this documented in the README's Supervisor Configuration so that users can easily learn how to change log level etc.

[github-actions]

Pinging code owners:

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

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[johannaojeling]

/label -needs-triage

[johannaojeling]

Self-assigning this