Delta exporters will Export only those points which received new update

[cijothomas]

Towards #2524 .
For delta, this only exports a point, if there is an update since last collection.
For cumulative, retains same behavior as before - once a point is updated, it'll be exported forever with same value.

This PR does not reclaim an inactive point for using when we are running out of metric points.

Perf impact:

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19043.1288 (21H1/May2021Update)
Intel Xeon CPU E5-1650 v4 3.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET SDK=6.0.100
[Host] : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT
DefaultJob : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT

Before:

Method	AggregationTemporality	Mean	Error	StdDev	Allocated
CounterHotPath	Cumulative	21.75 ns	0.363 ns	0.339 ns	-
CounterWith1LabelsHotPath	Cumulative	100.80 ns	1.198 ns	1.120 ns	-
CounterWith3LabelsHotPath	Cumulative	493.94 ns	9.660 ns	11.125 ns	-
CounterWith5LabelsHotPath	Cumulative	743.28 ns	12.839 ns	20.363 ns	-
CounterWith6LabelsHotPath	Cumulative	896.90 ns	14.747 ns	11.513 ns	-
CounterWith7LabelsHotPath	Cumulative	1,010.37 ns	18.028 ns	15.054 ns	-
CounterHotPath	Delta	19.32 ns	0.170 ns	0.133 ns	-
CounterWith1LabelsHotPath	Delta	102.44 ns	0.920 ns	0.768 ns	-
CounterWith3LabelsHotPath	Delta	500.14 ns	9.894 ns	11.778 ns	-
CounterWith5LabelsHotPath	Delta	751.90 ns	11.629 ns	11.942 ns	-
CounterWith6LabelsHotPath	Delta	870.99 ns	16.727 ns	17.897 ns	-
CounterWith7LabelsHotPath	Delta	995.99 ns	18.730 ns	23.003 ns	-

With this PR:

Method	AggregationTemporality	Mean	Error	StdDev	Allocated
CounterHotPath	Cumulative	20.44 ns	0.395 ns	0.422 ns	-
CounterWith1LabelsHotPath	Cumulative	102.18 ns	1.442 ns	1.349 ns	-
CounterWith3LabelsHotPath	Cumulative	515.90 ns	9.947 ns	9.305 ns	-
CounterWith5LabelsHotPath	Cumulative	760.88 ns	14.142 ns	12.536 ns	-
CounterWith6LabelsHotPath	Cumulative	891.03 ns	10.911 ns	9.111 ns	-
CounterWith7LabelsHotPath	Cumulative	1,044.78 ns	9.312 ns	8.711 ns	-
CounterHotPath	Delta	19.89 ns	0.344 ns	0.354 ns	-
CounterWith1LabelsHotPath	Delta	102.00 ns	0.646 ns	0.573 ns	-
CounterWith3LabelsHotPath	Delta	481.05 ns	8.155 ns	7.230 ns	-
CounterWith5LabelsHotPath	Delta	759.45 ns	11.807 ns	11.044 ns	-
CounterWith6LabelsHotPath	Delta	881.06 ns	11.320 ns	10.035 ns	-
CounterWith7LabelsHotPath	Delta	1,037.07 ns	20.667 ns	26.873 ns	-

(there is room for optimization to avoid the Delta vs Cumulative check on every Collect, as temporality is fixed at ctor itself. Good candidate for a small follow up PR)

Stress test:
Main:
Running (concurrency = 8), press to stop...
Stopping the stress test...

• Total Loops: 216,780,099
• Average Loops/Second: 7,037,858
• Average CPU Cycles/Loop: 3,953

With this PR:
Stopping the stress test...

• Total Loops: 204,110,887
• Average Loops/Second: 6,808,235
• Average CPU Cycles/Loop: 3,942

[cijothomas]

This is almost same as what @alanwest did the Metric Point clean up PR, except that this PR does not do the actual clean up/reuse.

[codecov]

Codecov Report

Merging #2629 (f95f2af) into main (5325185) will increase coverage by 0.01%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main    #2629      +/-   ##
==========================================
+ Coverage   79.83%   79.84%   +0.01%     
==========================================
  Files         249      249              
  Lines        8627     8659      +32     
==========================================
+ Hits         6887     6914      +27     
- Misses       1740     1745       +5     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/MetricPoint.cs	90.75% <72.22%> (-4.30%)	⬇️
src/OpenTelemetry/Metrics/AggregatorStore.cs	86.80% <100.00%> (+1.19%)	⬆️
src/OpenTelemetry/Metrics/BatchMetricPoint.cs	84.61% <100.00%> (+0.61%)	⬆️
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	93.03% <100.00%> (+0.04%)	⬆️
src/OpenTelemetry/Metrics/Metric.cs	95.77% <100.00%> (-0.06%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+5.88%)	⬆️

[reyang]

LGTM. Consider updating the changelog.

[alanwest]

👍