Metrics: MetricPointValueStorage explicit layout struct part2 (#2718)

src/OpenTelemetry/Metrics/HistogramBucket.cs

@@ -16,6 +16,9 @@
 
 namespace OpenTelemetry.Metrics
 {
+    /// <summary>
+    /// Represents a bucket in the histogram metric type.
+    /// </summary>
     public readonly struct HistogramBucket
     {
         internal HistogramBucket(double explicitBound, long bucketCount)
@@ -24,8 +27,15 @@ internal HistogramBucket(double explicitBound, long bucketCount)
             this.BucketCount = bucketCount;
         }
 
+        /// <summary>
+        /// Gets the configured bounds for the bucket or <see
+        /// cref="double.PositiveInfinity"/> for the catch-all bucket.
+        /// </summary>
         public double ExplicitBound { get; }
 
+        /// <summary>
+        /// Gets the count of items in the bucket.
+        /// </summary>
         public long BucketCount { get; }
     }
 }

src/OpenTelemetry/Metrics/HistogramBuckets.cs

@@ -16,26 +16,37 @@
 
 namespace OpenTelemetry.Metrics
 {
+    /// <summary>
+    /// A collection of <see cref="HistogramBucket"/>s associated with a histogram metric type.
+    /// </summary>
+    // Note: Does not implement IEnumerable<> to prevent accidental boxing.
     public class HistogramBuckets
     {
-        internal readonly long[] BucketCounts;
+        internal readonly double[] ExplicitBounds;
 
-        internal readonly long[] AggregatedBucketCounts;
+        internal readonly long[] RunningBucketCounts;
 
-        internal readonly double[] ExplicitBounds;
+        internal readonly long[] SnapshotBucketCounts;
 
-        internal readonly object LockObject;
+        internal double RunningSum;
 
-        internal HistogramBuckets(double[] histogramBounds)
+        internal double SnapshotSum;
+
+        internal HistogramBuckets(double[] explicitBounds)
         {
-            this.ExplicitBounds = histogramBounds;
-            this.BucketCounts = histogramBounds != null ? new long[histogramBounds.Length + 1] : null;
-            this.AggregatedBucketCounts = histogramBounds != null ? new long[histogramBounds.Length + 1] : null;
-            this.LockObject = new object();
+            this.ExplicitBounds = explicitBounds;
+            this.RunningBucketCounts = explicitBounds != null ? new long[explicitBounds.Length + 1] : null;
+            this.SnapshotBucketCounts = explicitBounds != null ? new long[explicitBounds.Length + 1] : new long[0];
         }
 
+        internal object LockObject => this.SnapshotBucketCounts;
+
         public Enumerator GetEnumerator() => new(this);
 
+        /// <summary>
+        /// Enumerates the elements of a <see cref="HistogramBuckets"/>.
+        /// </summary>
+        // Note: Does not implement IEnumerator<> to prevent accidental boxing.
         public struct Enumerator
         {
             private readonly int numberOfBuckets;
@@ -47,19 +58,30 @@ internal Enumerator(HistogramBuckets histogramMeasurements)
                 this.histogramMeasurements = histogramMeasurements;
                 this.index = 0;
                 this.Current = default;
-                this.numberOfBuckets = histogramMeasurements.AggregatedBucketCounts == null ? 0 : histogramMeasurements.AggregatedBucketCounts.Length;
+                this.numberOfBuckets = histogramMeasurements.SnapshotBucketCounts.Length;
             }
 
+            /// <summary>
+            /// Gets the <see cref="HistogramBucket"/> at the current position of the enumerator.
+            /// </summary>
             public HistogramBucket Current { get; private set; }
 
+            /// <summary>
+            /// Advances the enumerator to the next element of the <see
+            /// cref="HistogramBuckets"/>.
+            /// </summary>
+            /// <returns><see langword="true"/> if the enumerator was
+            /// successfully advanced to the next element; <see
+            /// langword="false"/> if the enumerator has passed the end of the
+            /// collection.</returns>
             public bool MoveNext()
             {
                 if (this.index < this.numberOfBuckets)
                 {
                     double explicitBound = this.index < this.numberOfBuckets - 1
                         ? this.histogramMeasurements.ExplicitBounds[this.index]
                         : double.PositiveInfinity;
-                    long bucketCount = this.histogramMeasurements.AggregatedBucketCounts[this.index];
+                    long bucketCount = this.histogramMeasurements.SnapshotBucketCounts[this.index];
                     this.Current = new HistogramBucket(explicitBound, bucketCount);
                     this.index++;
                     return true;

src/OpenTelemetry/Metrics/Metric.cs

@@ -23,7 +23,8 @@ namespace OpenTelemetry.Metrics
     public sealed class Metric
     {
         internal static readonly double[] DefaultHistogramBounds = new double[] { 0, 5, 10, 25, 50, 75, 100, 250, 500, 1000 };
-        private AggregatorStore aggStore;
+
+        private readonly AggregatorStore aggStore;
 
         internal Metric(
             Instrument instrument,
@@ -38,7 +39,8 @@ internal Metric(
             this.Description = metricDescription ?? string.Empty;
             this.Unit = instrument.Unit ?? string.Empty;
             this.Meter = instrument.Meter;
-            AggregationType aggType = default;
+
+            AggregationType aggType;
             if (instrument.GetType() == typeof(ObservableCounter<long>)
                 || instrument.GetType() == typeof(ObservableCounter<int>)
                 || instrument.GetType() == typeof(ObservableCounter<short>)