histogram: Add a no-op span for an otherwise empty histogram

Fixes #1127.

If a native histogram has no observations and a zero threshold of
zero, then it is indistinguishable from a classic histogram. To give
scrapers a hint that it is indeed a native histogram, we add a no-op
span.

This needs follow-up PRs in prometheus/prometheus and
prometheus/client_model.

Signed-off-by: beorn7 <[email protected]>

Author: beorn7

prometheus/histogram.go

@@ -787,6 +787,16 @@ func (h *histogram) Write(out *dto.Metric) error {
 		his.ZeroCount = proto.Uint64(zeroBucket)
 		his.NegativeSpan, his.NegativeDelta = makeBuckets(&coldCounts.nativeHistogramBucketsNegative)
 		his.PositiveSpan, his.PositiveDelta = makeBuckets(&coldCounts.nativeHistogramBucketsPositive)
+
+		// Add a no-op span to a histogram without observations and with
+		// a zero threshold of zero. Otherwise, a native histogram would
+		// look like a classic histogram to scrapers.
+		if *his.ZeroThreshold == 0 && *his.ZeroCount == 0 && len(his.PositiveSpan) == 0 && len(his.NegativeSpan) == 0 {
+			his.PositiveSpan = []*dto.BucketSpan{{
+				Offset: proto.Int32(0),
+				Length: proto.Uint32(0),
+			}}
+		}
 	}
 	addAndResetCounts(hotCounts, coldCounts)
 	return nil

prometheus/histogram_test.go

@@ -485,6 +485,17 @@ func TestNativeHistogram(t *testing.T) {
 			factor:       1,
 			want:         `sample_count:3 sample_sum:6 bucket:<cumulative_count:0 upper_bound:0.005 > bucket:<cumulative_count:0 upper_bound:0.01 > bucket:<cumulative_count:0 upper_bound:0.025 > bucket:<cumulative_count:0 upper_bound:0.05 > bucket:<cumulative_count:0 upper_bound:0.1 > bucket:<cumulative_count:0 upper_bound:0.25 > bucket:<cumulative_count:0 upper_bound:0.5 > bucket:<cumulative_count:1 upper_bound:1 > bucket:<cumulative_count:2 upper_bound:2.5 > bucket:<cumulative_count:3 upper_bound:5 > bucket:<cumulative_count:3 upper_bound:10 > `, // Has conventional buckets because there are no sparse buckets.
 		},
+		{
+			name:   "no observations",
+			factor: 1.1,
+			want:   `sample_count:0 sample_sum:0 schema:3 zero_threshold:2.938735877055719e-39 zero_count:0 `,
+		},
+		{
+			name:          "no observations and zero threshold of zero resulting in no-op span",
+			factor:        1.1,
+			zeroThreshold: NativeHistogramZeroThresholdZero,
+			want:          `sample_count:0 sample_sum:0 schema:3 zero_threshold:0 zero_count:0 positive_span:<offset:0 length:0 > `,
+		},
 		{
 			name:         "factor 1.1 results in schema 3",
 			observations: []float64{0, 1, 2, 3},