Merge pull request #175 from prometheus/beorn7/histogram

Fix issue with histogram rendering

README.md

@@ -82,9 +82,11 @@ Example input from stdin:
 Note that all numbers are encoded as strings. Some parsers want it
 that way. Also, Prometheus allows sample values like `NaN` or `+Inf`,
 which cannot be encoded as JSON numbers.
-Native histograms are formated similarly as [the query 
-API](https://prometheus.io/docs/prometheus/latest/querying/api/#native-histograms) 
-would return.
+
+A histogram is formatted as a native histogram if it has at least one span. It
+is then formatted in a similar way as [the Prometehus query
+API](https://prometheus.io/docs/prometheus/latest/querying/api/#native-histograms)
+does it.
 
 ```json
 [
@@ -136,7 +138,7 @@ would return.
   {
     "name": "http_request_duration_seconds",
     "type": "HISTOGRAM",
-    "help": "More HTTP request latencies in seconds.",
+    "help": "This is a native histogram.",
     "metrics": [
       {
         "labels": {
@@ -166,6 +168,39 @@ would return.
         "sum": "29969.50000000001"
       }
     ]
+  },
+  {
+    "name": "some_weird_normal_distribution",
+    "type": "HISTOGRAM",
+    "help": "This is a classic histogram.",
+    "metrics": [
+      {
+        "buckets": {
+          "-0.0001899999999999998": "17",
+          "-0.0002899999999999998": "6",
+          "-0.0003899999999999998": "2",
+          "-0.0004899999999999998": "2",
+          "-0.0005899999999999998": "0",
+          "-0.0006899999999999999": "0",
+          "-0.0007899999999999999": "0",
+          "-0.00089": "0",
+          "-0.00099": "0",
+          "-8.999999999999979e-05": "33",
+          "0.00011000000000000022": "75",
+          "0.00021000000000000023": "92",
+          "0.0003100000000000002": "100",
+          "0.0004100000000000002": "103",
+          "0.0005100000000000003": "105",
+          "0.0006100000000000003": "106",
+          "0.0007100000000000003": "107",
+          "0.0008100000000000004": "107",
+          "0.0009100000000000004": "107",
+          "1.0000000000000216e-05": "50"
+        },
+        "count": "107",
+        "sum": "0.001792103516591124"
+      }
+    ]
   }
 ]
 ```

prom2json.go

@@ -108,22 +108,24 @@ func getValue(m *dto.Metric) float64 {
 }
 
 func makeHistogram(m *dto.Metric) Histogram {
+	dtoH := m.GetHistogram()
 	hist := Histogram{
 		Labels:      makeLabels(m),
 		TimestampMs: makeTimestamp(m),
-		Count:       fmt.Sprint(m.GetHistogram().GetSampleCount()),
-		Sum:         fmt.Sprint(m.GetHistogram().GetSampleSum()),
+		Count:       fmt.Sprint(dtoH.GetSampleCount()),
+		Sum:         fmt.Sprint(dtoH.GetSampleSum()),
 	}
-	if b := makeBuckets(m); len(b) > 0 {
-		hist.Buckets = b
-	} else {
-		h, fh := histogram.NewModelHistogram(m.GetHistogram())
+	// A native histogram is marked by at least one span.
+	if len(dtoH.GetNegativeSpan())+len(dtoH.GetPositiveSpan()) > 0 {
+		h, fh := histogram.NewModelHistogram(dtoH)
 		if h == nil {
 			// float histogram
 			hist.Buckets = histogram.BucketsAsJson[float64](histogram.GetAPIFloatBuckets(fh))
 		} else {
 			hist.Buckets = histogram.BucketsAsJson[uint64](histogram.GetAPIBuckets(h))
 		}
+	} else {
+		hist.Buckets = makeBuckets(m)
 	}
 	return hist
 }