[sdk-metrics] Improve the perf of histogram snapshots

[CodeBlanch]

Changes

• For histograms with buckets switch from two arrays (one for running values and one for snapshot values) to a single array containing a structure with both values. This speeds up the snapshot logic because we can now elide bounds checking. That is important because when it comes to histograms the snapshot blocks the recording of values. Making it faster reduces contention which increases throughput when a collection occurs.

Benchmarks

Method	OutputDelta	Mean	Error	StdDev
UpdateOld	False	0.2130 ns	0.0222 ns	0.0208 ns
UpdateNew	False	0.1810 ns	0.0264 ns	0.0247 ns
SnapshotOld	False	9.8634 ns	0.1283 ns	0.1200 ns
SnapshotNew	False	4.0386 ns	0.0894 ns	0.0836 ns
UpdateOld	True	0.1948 ns	0.0247 ns	0.0231 ns
UpdateNew	True	0.1815 ns	0.0120 ns	0.0107 ns
SnapshotOld	True	10.7866 ns	0.2127 ns	0.1885 ns
SnapshotNew	True	7.3044 ns	0.1561 ns	0.1603 ns

Code

public class HistogramSnapshotBenchmarks
{
    private readonly HistogramBucketValues[] bucketCounts = new HistogramBucketValues[16];
    private readonly long[] runningBucketCounts = new long[16];
    private readonly long[] snapshotBucketCounts = new long[16];

    [Params(true, false)]
    public bool OutputDelta { get; set; }

    [Benchmark]
    public long UpdateOld()
    {
        return this.runningBucketCounts[0]++;
    }

    [Benchmark]
    public long UpdateNew()
    {
        return this.bucketCounts[0].RunningValue++;
    }

    [Benchmark]
    public void SnapshotOld()
    {
        for (int i = 0; i < this.runningBucketCounts.Length; i++)
        {
            this.snapshotBucketCounts[i] = this.runningBucketCounts[i];
            if (this.OutputDelta)
            {
                this.runningBucketCounts[i] = 0;
            }
        }
    }

    [Benchmark]
    public void SnapshotNew()
    {
        var bucketCounts = this.bucketCounts;

        if (this.OutputDelta)
        {
            for (int i = 0; i < bucketCounts.Length; i++)
            {
                ref var values = ref bucketCounts[i];
                ref var running = ref values.RunningValue;
                values.SnapshotValue = running;
                running = 0L;
            }
        }
        else
        {
            for (int i = 0; i < bucketCounts.Length; i++)
            {
                ref var values = ref bucketCounts[i];
                values.SnapshotValue = values.RunningValue;
            }
        }
    }

    internal struct HistogramBucketValues
    {
        public long RunningValue;
        public long SnapshotValue;
    }
}

## Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (6250307) 83.38% compared to head (805ccb5) 83.13%.
Report is 85 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5366      +/-   ##
==========================================
- Coverage   83.38%   83.13%   -0.26%     
==========================================
  Files         297      276      -21     
  Lines       12531    12001     -530     
==========================================
- Hits        10449     9977     -472     
+ Misses       2082     2024      -58     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	82.88% <95.23%> (?)
unittests-Solution-Stable	83.13% <95.23%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/OpenTelemetry/Metrics/HistogramBuckets.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/MetricPoint.cs	68.45% <83.33%> (-0.02%)	⬇️

... and 45 files with indirect coverage changes