Refactor for new [Gauge|Sum|Summary|Histogram]Metric types and aggregators

[victlu]

• Define IGaugeMetric, ISumMetric, ISummaryMetric, and IHistogramMetric
• Refactor processor pipeline to handle aggregators (aka IMetricBuilder).
• Refactor AggregatorStore to allow for multiple exporters with different CollectionPeriods.

[victlu]

FYI: @CodeBlanch @alanwest @utpilla

[codecov]

Codecov Report

Merging #2073 (786c636) into metrics (23987a4) will decrease coverage by 1.42%.
The diff coverage is 57.96%.

@@             Coverage Diff             @@
##           metrics    #2073      +/-   ##
===========================================
- Coverage    83.99%   82.57%   -1.43%     
===========================================
  Files          213      214       +1     
  Lines         6586     6776     +190     
===========================================
+ Hits          5532     5595      +63     
- Misses        1054     1181     +127     

Impacted Files	Coverage Δ
...rc/OpenTelemetry/Metrics/DataPoint/DataPoint{T}.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/Exemplar.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/Exemplar{T}.cs	0.00% <0.00%> (ø)
...trics/MetricAggregators/SummaryMetricAggregator.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/DataPoint.cs	37.50% <42.85%> (ø)
...Metrics/MetricAggregators/GaugeMetricAggregator.cs	75.00% <75.00%> (ø)
...y/Metrics/MetricAggregators/SumMetricAggregator.cs	80.00% <80.00%> (ø)
src/OpenTelemetry/Metrics/AggregatorStore.cs	88.57% <90.24%> (+0.43%)	⬆️
...ics/MetricAggregators/HistogramMetricAggregator.cs	97.36% <97.36%> (ø)
src/OpenTelemetry/Metrics/InstrumentState.cs	100.00% <100.00%> (ø)
... and 16 more

[alanwest]

LGTM understanding that things will likely to continue to evolve