histogram: Identify native histograms even without observations

Native histograms without observations and with a zero threshold of
zero look the same as classic histograms in the protobuf exposition
format. According to
prometheus/client_golang#1127 , the idea is
to add a no-op span to those histograms to mark them as native
histograms. This commit enables Prometheus to detect that no-op span
and adds a doc comment to the proto spec describing the behavior.

Signed-off-by: beorn7 <beorn@grafana.com>

model/textparse/protobufparse.go

@@ -554,20 +554,17 @@ func formatOpenMetricsFloat(f float64) string {
 	return s + ".0"
 }
 
-// isNativeHistogram returns false iff the provided histograms has no sparse
-// buckets and a zero threshold of 0 and a zero count of 0. In principle, this
-// could still be meant to be a native histogram (with a zero threshold of 0 and
-// no observations yet), but for now, we'll treat this case as a conventional
-// histogram.
-//
-// TODO(beorn7): In the final format, there should be an unambiguous way of
-// deciding if a histogram should be ingested as a conventional one or a native
-// one.
+// isNativeHistogram returns false iff the provided histograms has no spans at
+// all (neither positive nor negative) and a zero threshold of 0 and a zero
+// count of 0. In principle, this could still be meant to be a native histogram
+// with a zero threshold of 0 and no observations yet. In that case,
+// instrumentation libraries should add a "no-op" span (e.g. length zero, offset
+// zero) to signal that the histogram is meant to be parsed as a native
+// histogram. Failing to do so will cause Prometheus to parse it as a classic
+// histogram as long as no observations have happened.
 func isNativeHistogram(h *dto.Histogram) bool {
-	return h.GetZeroThreshold() > 0 ||
-		h.GetZeroCount() > 0 ||
-		len(h.GetNegativeDelta()) > 0 ||
-		len(h.GetPositiveDelta()) > 0 ||
-		len(h.GetNegativeCount()) > 0 ||
-		len(h.GetPositiveCount()) > 0
+	return len(h.GetPositiveSpan()) > 0 ||
+		len(h.GetNegativeSpan()) > 0 ||
+		h.GetZeroThreshold() > 0 ||
+		h.GetZeroCount() > 0
 }

model/textparse/protobufparse_test.go

@@ -517,6 +517,19 @@ metric: <
     sample_sum: 1.234
   >
 >
+`,
+		`name: "empty_histogram"
+help: "A histogram without observations and with a zero threshold of zero but with a no-op spam to identify it as a native histogram."
+type: HISTOGRAM
+metric: <
+  histogram: <
+    positive_span: <
+      offset: 0
+      length: 0
+    >
+  >
+>
+
 `,
 	}
 
@@ -965,6 +978,25 @@ func TestProtobufParse(t *testing.T) {
 						"__name__", "without_quantiles_sum",
 					),
 				},
+				{
+					m:    "empty_histogram",
+					help: "A histogram without observations and with a zero threshold of zero but with a no-op spam to identify it as a native histogram.",
+				},
+				{
+					m:   "empty_histogram",
+					typ: MetricTypeHistogram,
+				},
+				{
+					m: "empty_histogram",
+					shs: &histogram.Histogram{
+						CounterResetHint: histogram.UnknownCounterReset,
+						PositiveSpans:    []histogram.Span{},
+						NegativeSpans:    []histogram.Span{},
+					},
+					lset: labels.FromStrings(
+						"__name__", "empty_histogram",
+					),
+				},
 			},
 		},
 		{
@@ -1688,6 +1720,25 @@ func TestProtobufParse(t *testing.T) {
 						"__name__", "without_quantiles_sum",
 					),
 				},
+				{ // 78
+					m:    "empty_histogram",
+					help: "A histogram without observations and with a zero threshold of zero but with a no-op spam to identify it as a native histogram.",
+				},
+				{ // 79
+					m:   "empty_histogram",
+					typ: MetricTypeHistogram,
+				},
+				{ // 80
+					m: "empty_histogram",
+					shs: &histogram.Histogram{
+						CounterResetHint: histogram.UnknownCounterReset,
+						PositiveSpans:    []histogram.Span{},
+						NegativeSpans:    []histogram.Span{},
+					},
+					lset: labels.FromStrings(
+						"__name__", "empty_histogram",
+					),
+				},
 			},
 		},
 	}

prompb/io/prometheus/client/metrics.proto

@@ -97,6 +97,9 @@ message Histogram {
   repeated double negative_count = 11; // Absolute count of each bucket.
 
   // Positive buckets for the native histogram.
+  // Use a no-op span (offset 0, length 0) for a native histogram without any
+  // observations yet and with a zero_threshold of 0. Otherwise, it would be
+  // indistinguishable from a classic histogram.
   repeated BucketSpan positive_span = 12 [(gogoproto.nullable) = false];
   // Use either "positive_delta" or "positive_count", the former for
   // regular histograms with integer counts, the latter for float