Metrics: MetricPointValueStorage explicit layout struct part2

[CodeBlanch]

Changes

• Followed-up on conversation from the last PR. Moved the histogram sum tracking to HistogramBuckets. That saved 8 bytes for the non-histogram MetricPoints. Also used SnapshotBucketCounts for the histogram lock to save an object allocation.

• Added missing xml comments in MetricPoint, HistogramBuckets, & HistogramBucket.

• Added some inlining hints on MetricPoint.

• Renamed from "current"/"snapshot" to "running"/"snapshot" which seemed closer to what is going on.

Benchmarks

Before

AggType	Allocated
LongSumIncomingCumulative	112 B
DoubleSumIncomingCumulative	112 B
Histogram	408 B

After

AggType	Allocated
LongSumIncomingCumulative	104 B
DoubleSumIncomingCumulative	104 B
Histogram	384 B

Investigation Notes

What I really wanted to do was this (more or less):

    [StructLayout(LayoutKind.Explicit)]
    internal struct MetricPointSecondaryValueStorage
    {
        [FieldOffset(0)]
        public double DeltaLastValue;

        [FieldOffset(0)]
        public HistogramBuckets HistogramBuckets;
    }

Basically use 8 bytes for either the delta value or the pointer to HistogramBuckets. Actually compiles fine, but blows up trying to load the assembly. Something about you can't overlap a reference object with a value type because it would mess up the GC's ability to track that reference in order to know if it should be cleaned up or not.

[codecov]

Codecov Report

Merging #2718 (400fec9) into main (4bf8ed4) will decrease coverage by 0.00%.
The diff coverage is 82.89%.

@@            Coverage Diff             @@
##             main    #2718      +/-   ##
==========================================
- Coverage   83.77%   83.76%   -0.01%     
==========================================
  Files         251      251              
  Lines        8830     8835       +5     
==========================================
+ Hits         7397     7401       +4     
- Misses       1433     1434       +1     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/HistogramBucket.cs	100.00% <ø> (ø)
src/OpenTelemetry/Metrics/Metric.cs	94.44% <ø> (-0.08%)	⬇️
src/OpenTelemetry/Metrics/MetricPoint.cs	85.60% <81.15%> (-0.11%)	⬇️
src/OpenTelemetry/Metrics/HistogramBuckets.cs	100.00% <100.00%> (ø)
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	70.75% <0.00%> (-0.95%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+2.94%)	⬆️

[mic-max]

LGTM 👍