[Metrics API/SDK] - Support multiple Instrument callbacks

[lalitb]

As per the specs -

"The Metrics API MAY support an interface allowing the use of multiple instruments from a single registered Callback."

As of now, implementation supports registering multiple callbacks to single instruments. While specs also optionally allows registering single callback with multiple instruments (so multiple measurements can be returned from single callback functions).

[github-actions]

This issue was marked as stale due to lack of activity.

[marcalff]

Adding the breaking change label: this will require a new API, hence changing the API ABI for metrics.

[marcalff]

Supporting multiple instruments in a single callback is valuable when an expensive computation is needed to get the data.

A typical example is reporting multiple instruments at once when reading a /proc file.

[KJTsanaktsidis]

I recently ran into the need for this API. Is there anything blocking this being implemented in the V2 ABI? Or is this just waiting for someone to open a PR with a proposed implementation to iterate on?