Adds a cache to the metric insrument pipeline.

[MadVikingGod]

This adds a caching layer to instrument creation, so repeated requests for the same instrument (scope, name, kind) return the same underlying aggregations.

Fix #3229

[codecov]

Codecov Report

Merging #3181 (a316885) into main (b6d4335) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3181   +/-   ##
=====================================
  Coverage   77.4%   77.5%           
=====================================
  Files        158     158           
  Lines      11084   11139   +55     
=====================================
+ Hits        8583    8636   +53     
- Misses      2306    2308    +2     
  Partials     195     195           

Impacted Files	Coverage Δ
sdk/metric/instrument_provider.go	80.0% <100.0%> (ø)
sdk/metric/pipeline.go	97.9% <100.0%> (+0.6%)	⬆️
sdk/trace/batch_span_processor.go	80.2% <0.0%> (-0.9%)	⬇️

[MrAlias]

I think this approach is viable in that it is should provide correct behavior, but I think it suffers from two issues:

1. this expands on the confusion of multiple "create" methods as talked about in Refactor Pipeline #3233
2. it overloads the pipelineRegistry so now it is now a pipeline registry and instrument cache

The first issue I feel is sympomatic of the current pipeline design and it would be addressed in #3233. The second though, I feel needs to be addressed by moving the cache to the Meter object. That is where instrument uniqueness is defined from and could be constructed to emanate from that.

I built out a PoC based on #3233 with how we could resolve both of these issues: https://github.com/MrAlias/opentelemetry-go/pull/660/files

[MrAlias]

Suggested change

	- Add caching to instrument creation (#3181)
	- Add caching to instrument creation.
	Repeated requests for the same instrument will return the same instance. (#3181)

[MrAlias]

Closing as this has been superseded by #3251

Please reopen if this is in error.