Exponential Bucket Histogram - part 7 (#3499)

* hammer out the upper/lower bound of scale

* comment

src/OpenTelemetry/Metrics/ExponentialBucketHistogram.cs

@@ -35,7 +35,18 @@ internal class ExponentialBucketHistogram
     private int scale;
     private double scalingFactor; // 2 ^ scale / log(2)
 
-    public ExponentialBucketHistogram(int scale, int maxBuckets = 160)
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ExponentialBucketHistogram"/> class.
+    /// </summary>
+    /// <param name="maxBuckets">
+    /// The maximum number of buckets in each of the positive and negative ranges, not counting the special zero bucket. The default value is 160.
+    /// </param>
+    public ExponentialBucketHistogram(int maxBuckets = 160)
+        : this(maxBuckets, 20)
+    {
+    }
+
+    internal ExponentialBucketHistogram(int maxBuckets, int scale)
     {
         /*
         The following table is calculated based on [ MapToIndex(double.Epsilon), MapToIndex(double.MaxValue) ]:
@@ -80,7 +91,12 @@ The following table is calculated based on [ MapToIndex(double.Epsilon), MapToIn
         */
         Guard.ThrowIfOutOfRange(scale, min: -11, max: 20);
 
-        Guard.ThrowIfOutOfRange(maxBuckets, min: 1);
+        /*
+        Regardless of the scale, MapToIndex(1) will always be -1, so we need two buckets at minimum:
+            bucket[-1] = (1/base, 1]
+            bucket[0] = (1, base]
+        */
+        Guard.ThrowIfOutOfRange(maxBuckets, min: 2);
 
         this.Scale = scale;
         this.PositiveBuckets = new CircularBufferBuckets(maxBuckets);