Fix cumulative histogram generation and test.

Issue google#214

Author: jaqx0r

internal/metrics/datum/buckets_test.go

@@ -4,6 +4,7 @@
 package datum_test
 
 import (
+	"math"
 	"testing"
 	"testing/quick"
 	"time"
@@ -36,4 +37,8 @@ func TestMakeBucket(t *testing.T) {
 	if r := datum.GetBucketsCount(b); r != 1 {
 		t.Errorf("count not 1, got %v", r)
 	}
+	bs := datum.GetBucketsByMax(b)
+	if r := datum.GetBucketsCount(b); r != bs[math.Inf(+1)] {
+		t.Errorf("Inf bucket des not equal total observation count: %v vs %v", r, bs[math.Inf(+1)])
+	}
 }

internal/metrics/datum/datum.go

@@ -262,13 +262,16 @@ func GetBucketsSum(d Datum) float64 {
 	}
 }
 
-// GetBucketsByMax returns a map of bucket observations by their upper bonds, or panics if d is not a BucketsDatum
+// GetBucketsByMax returns a map of cumulative bucket observations by their
+// upper bonds, or panics if d is not a BucketsDatum.
 func GetBucketsByMax(d Datum) map[float64]uint64 {
 	switch d := d.(type) {
 	case *BucketsDatum:
 		buckets := make(map[float64]uint64)
+		cum := uint64(0)
 		for r, c := range d.Buckets() {
-			buckets[r.Max] = c
+			cum += c
+			buckets[r.Max] = cum
 		}
 		return buckets
 	default: