{summary,histogram}: properly addTags to empty labels set

Metric name with explicitly set empty labels set is valid syntax, for example:
`metric_name{}`.

 This case works fine for all metric types except histogram and summary.
Since it adds own tags via addTag function.

 addTag function incorrectly handles empty labels set and adds extra comma:
`metric_name{,quantile="0.1"}`. It violites prometheus text exposition format.

 This commit adds extra check for addTag function and fixes this issue.

Related issue:
#78

Signed-off-by: f41gh7 <[email protected]>

histogram_test.go

@@ -200,6 +200,20 @@ func TestHistogramWithTags(t *testing.T) {
 	}
 }
 
+func TestHistogramWithEmptyTags(t *testing.T) {
+	name := `TestHistogram{}`
+	h := NewHistogram(name)
+	h.Update(123)
+
+	var bb bytes.Buffer
+	WritePrometheus(&bb, false)
+	result := bb.String()
+	namePrefixWithTag := `TestHistogram_bucket{vmrange="1.136e+02...1.292e+02"} 1` + "\n"
+	if !strings.Contains(result, namePrefixWithTag) {
+		t.Fatalf("missing histogram %s in the WritePrometheus output; got\n%s", namePrefixWithTag, result)
+	}
+}
+
 func TestGetOrCreateHistogramSerial(t *testing.T) {
 	name := "GetOrCreateHistogramSerial"
 	if err := testGetOrCreateHistogram(name); err != nil {

summary.go

@@ -208,7 +208,15 @@ func addTag(name, tag string) string {
 	if len(name) == 0 || name[len(name)-1] != '}' {
 		return fmt.Sprintf("%s{%s}", name, tag)
 	}
-	return fmt.Sprintf("%s,%s}", name[:len(name)-1], tag)
+	name = name[:len(name)-1]
+	if len(name) == 0 {
+		panic(fmt.Errorf("BUG: metric name cannot be empty"))
+	}
+	if name[len(name)-1] == '{' {
+		// case for empty labels set metric_name{}
+		return fmt.Sprintf("%s%s}", name, tag)
+	}
+	return fmt.Sprintf("%s,%s}", name, tag)
 }
 
 func registerSummaryLocked(sm *Summary) {

summary_test.go

@@ -77,6 +77,20 @@ func TestSummaryWithTags(t *testing.T) {
 	}
 }
 
+func TestSummaryWithEmptyTags(t *testing.T) {
+	name := `TestSummary{}`
+	s := NewSummary(name)
+	s.Update(123)
+
+	var bb bytes.Buffer
+	WritePrometheus(&bb, false)
+	result := bb.String()
+	namePrefixWithTag := `TestSummary{quantile="`
+	if !strings.Contains(result, namePrefixWithTag) {
+		t.Fatalf("missing summary prefix %s in the WritePrometheus output; got\n%s", namePrefixWithTag, result)
+	}
+}
+
 func TestSummaryInvalidQuantiles(t *testing.T) {
 	name := "SummaryInvalidQuantiles"
 	expectPanic(t, name, func() {