From 6d084a014147a3e4283d5964d865e8396be6a64d Mon Sep 17 00:00:00 2001
From: Owen Smith <owen.rl.smith@gmail.com>
Date: Sat, 13 Jan 2024 23:44:06 +0000
Subject: [PATCH] Avoid off-spec openmetrics exposition when exemplars have
 empty labels

Will fix https://github.com/prometheus/client_golang/issues/1333

Signed-off-by: Owen Smith <owen.rl.smith@gmail.com>
---
 expfmt/openmetrics_create.go      |  2 +-
 expfmt/openmetrics_create_test.go | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/expfmt/openmetrics_create.go b/expfmt/openmetrics_create.go
index 21cdddcf..6f2c8986 100644
--- a/expfmt/openmetrics_create.go
+++ b/expfmt/openmetrics_create.go
@@ -350,7 +350,7 @@ func writeOpenMetricsSample(
 			return written, err
 		}
 	}
-	if exemplar != nil {
+	if exemplar != nil && len(exemplar.Label) > 0 {
 		n, err = writeExemplar(w, exemplar)
 		written += n
 		if err != nil {
diff --git a/expfmt/openmetrics_create_test.go b/expfmt/openmetrics_create_test.go
index ab1e8616..5fa25c78 100644
--- a/expfmt/openmetrics_create_test.go
+++ b/expfmt/openmetrics_create_test.go
@@ -417,6 +417,31 @@ foos_total 42.0
 			},
 			out: `# HELP name doc string
 # TYPE name counter
+`,
+		},
+		// 9: Simple Counter with exemplar that has empty label set:
+		// ignore the exemplar, since OpenMetrics spec requires labels.
+		{
+			in: &dto.MetricFamily{
+				Name: proto.String("foos_total"),
+				Help: proto.String("Number of foos."),
+				Type: dto.MetricType_COUNTER.Enum(),
+				Metric: []*dto.Metric{
+					{
+						Counter: &dto.Counter{
+							Value: proto.Float64(42),
+							Exemplar: &dto.Exemplar{
+								Label:     []*dto.LabelPair{},
+								Value:     proto.Float64(1),
+								Timestamp: openMetricsTimestamp,
+							},
+						},
+					},
+				},
+			},
+			out: `# HELP foos Number of foos.
+# TYPE foos counter
+foos_total 42.0
 `,
 		},
 	}