Metrics SDK - can we cleanup after a shutdown?

[cijothomas]

Though shutdown prevents metrics from ever getting collected/exported, they are still kept in the HashMaps/Aggregations. There is no check in hot path to see if the provider is already shutdown (it may be too expensive to do that check). This maybe okay, but opening an issue to discuss more.

Also, given the Counter/Meter from the API use a Arc<dyn> object pointing to something from the SDK, it is not easily possible to do much cleanup in the SDK, unless the user explicitly drops the Counter/Meters themselves.

[cijothomas]

@utpilla @fraillt FYI. Something we can discuss.

[fraillt]

As you have mentioned, I would prefer to avoid having this check in hot path as well.
However, we should probably clear Pipeline (more precisely inner: Mutex<PipelineInner>) during shutdown, so that memory can be cleaned up once user explicitly drops meters themselves.