fix(metrics): avoid duplicate buckets in Mid1To16k

Author: zawadzkidiana

common/metrics/histograms.go

@@ -60,9 +60,11 @@ var (
 
 	// Mid1To16k is a histogram for small counters, like "how many replication tasks did we receive".
 	//
-	// This targets 1 to ~10k with some buffer, and ends just below 64k.
-	// Do not rely on this for values which can ever reach 64k, make a new histogram.
-	Mid1To16k = IntSubsettableHistogram(makeSubsettableHistogram(2, 1, 16384, 64))
+	// This targets single-digits through ~10k with some buffer, and ends well below 1M.
+	//
+	// Note: we intentionally start at 8 (not 1). Starting at 1 causes duplicate bucket boundaries
+	// due to float->duration truncation, which Prometheus rejects.
+	Mid1To16k = IntSubsettableHistogram(makeSubsettableHistogram(2, 8, 16384, 64))
 )
 
 // SubsettableHistogram is a duration-based histogram that can be subset to a lower scale

common/metrics/histograms_test.go

@@ -130,20 +130,8 @@ func TestHistogramValues(t *testing.T) {
 `)
 	})
 	t.Run("mid_to_16k_ints", func(t *testing.T) {
-		// note: this histogram has some duplicates.
-		//
-		// this wastes a bit of memory, but Tally will choose the same index each
-		// time for a specific value, so it does not cause extra non-zero series
-		// to be stored or emitted.
-		//
-		// if this turns out to be expensive in Prometheus / etc, it's easy
-		// enough to start the histogram at 8, and dual-emit a non-exponential
-		// histogram for lower values.
 		checkHistogram(t, Mid1To16k, `
 [0]
-[1 1 1 1]
-[2 2 2 3]
-[4 4 5 6]
 [8 9 11 13]
 [16 19 22 26]
 [32 38 45 53]
@@ -157,7 +145,10 @@ func TestHistogramValues(t *testing.T) {
 [8192 9741 11585 13777]
 [16384 19483 23170 27554]
 [32768 38967 46340 55108]
-[65536]
+[65536 77935 92681 110217]
+[131072 155871 185363 220435]
+[262144 311743 370727 440871]
+[524288]
 `)
 	})
 }