Fix protobuf parsing of quantile-less summaries (#9277)

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

pkg/textparse/protobufparse.go

@@ -96,6 +96,10 @@ func (p *ProtobufParser) Series() ([]byte, *int64, float64) {
 			v = float64(s.GetSampleCount())
 		case -1:
 			v = s.GetSampleSum()
+			// Need to detect a summaries without quantile here.
+			if len(s.GetQuantile()) == 0 {
+				p.fieldsDone = true
+			}
 		default:
 			v = s.GetQuantile()[p.fieldPos].GetValue()
 		}

pkg/textparse/protobufparse_test.go

@@ -227,6 +227,16 @@ metric: <
     >
   >
 >
+`,
+		`name: "without_quantiles"
+help: "A summary without quantiles."
+type: SUMMARY
+metric: <
+  summary: <
+    sample_count: 42
+    sample_sum: 1.234
+  >
+>
 `,
 	}
 
@@ -458,6 +468,28 @@ metric: <
 				"service", "exponential",
 			),
 		},
+		{
+			m:    "without_quantiles",
+			help: "A summary without quantiles.",
+		},
+		{
+			m:   "without_quantiles",
+			typ: MetricTypeSummary,
+		},
+		{
+			m: "without_quantiles_count",
+			v: 42,
+			lset: labels.FromStrings(
+				"__name__", "without_quantiles_count",
+			),
+		},
+		{
+			m: "without_quantiles_sum",
+			v: 1.234,
+			lset: labels.FromStrings(
+				"__name__", "without_quantiles_sum",
+			),
+		},
 	}
 
 	p := NewProtobufParser(inputBuf.Bytes())