Add support for OpAmp throttling to opampextension #1712
Description
We want the Sumo opamp agent to respond appropriately when the server tells it to throttle.
When a ServerToAgent message is received with an error whose type is "UNAVAILBLE", the opamp client should disconnect.
https://opentelemetry.io/docs/specs/opamp/#throttling
The opamp-go library leaves it to us as the agent developers to actually implement that. We are just logging an error at the moment: https://github.com/SumoLogic/sumologic-otel-collector/blob/main/pkg/extension/opampextension/opamp_agent.go#L167
That callback needs to be replaced with one that disconnects the session, and retries it after the RetryAfter amount sent in the error message.