Initial framework for Aggregators

[victlu]

Introduce initial framework for plumbing in Aggregators into Metrics pipeline.

One goal is to avoid any allocations in the "Hot Path".

[codecov]

Codecov Report

Merging #2053 (b2433b7) into metrics (4190b6b) will decrease coverage by 0.03%.
The diff coverage is 81.37%.

@@             Coverage Diff             @@
##           metrics    #2053      +/-   ##
===========================================
- Coverage    82.94%   82.90%   -0.04%     
===========================================
  Files          216      224       +8     
  Lines         6560     6804     +244     
===========================================
+ Hits          5441     5641     +200     
- Misses        1119     1163      +44     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/Aggregator.cs	0.00% <0.00%> (ø)
src/System.Diagnostics.Metrics.Temp/Counter.cs	60.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint.cs	48.00% <45.83%> (-37.72%)	⬇️
src/System.Diagnostics.Metrics.Temp/InstrumentT.cs	72.22% <64.00%> (-4.71%)	⬇️
...enTelemetry/Metrics/ObjectArrayEqualityComparer.cs	73.33% <73.33%> (ø)
...enTelemetry/Metrics/StringArrayEqualityComparer.cs	73.33% <73.33%> (ø)
.../OpenTelemetry/Metrics/Aggregator/SumAggregator.cs	78.94% <78.94%> (ø)
src/OpenTelemetry/Metrics/DataPoint{T}.cs	85.71% <85.71%> (+42.85%)	⬆️
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	94.44% <86.48%> (-4.31%)	⬇️
src/OpenTelemetry/Metrics/AggregatorStore.cs	88.13% <88.13%> (ø)
... and 20 more

[tarekgh]

CC @noahfalk for awareness.

[CodeBlanch]

LGTM

[alanwest]

LGTM. Added just a few minor questions/comments that could easily be addressed later if needed.

[cijothomas]

Thanks!

Left some comments, but we'll keep iterating and improve this as we move forward. I am sure we can do lot more optimizations in the future.