Fix exemplars being added to gauge metrics in the prometheus exporter (#5912)

Prometheus Gauge metrics don't support exemplars and while
`addGaugeMetric()` doesn't add them, `addSumMetric()` will if the metric
is monotonic. This causes the prometheus client to throw an error:

```
* error collecting metric Desc{fqName: "http_server_request_body_size_bytes", help: "Measures size of RPC request messages (uncompressed).", constLabels: {}, variableLabels: {net_protocol_name,net_protocol_version,http_method,http_route,http_scheme,net_host_name,net_host_port,otel_scope_name,otel_scope_version}}: cannot inject exemplar into Gauge, Summary or Untyped
```

---------

Co-authored-by: Damien Mathieu <42@dmathieu.com>
Co-authored-by: David Ashpole <dashpole@google.com>

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Fixed
 
 - Global MeterProvider registration unwraps global instrument Observers, the undocumented Unwrap() methods are now private. (#5881)
+- Fix `go.opentelemetry.io/otel/exporters/prometheus` trying to add exemplars to Gauge metrics, which is unsupported. (#5912)
 
 ### Changed
 

exporters/prometheus/exporter.go

@@ -277,7 +277,11 @@ func addSumMetric[N int64 | float64](ch chan<- prometheus.Metric, sum metricdata
 			otel.Handle(err)
 			continue
 		}
-		m = addExemplars(m, dp.Exemplars)
+		// GaugeValues don't support Exemplars at this time
+		// https://github.com/prometheus/client_golang/blob/aef8aedb4b6e1fb8ac1c90790645169125594096/prometheus/metric.go#L199
+		if valueType != prometheus.GaugeValue {
+			m = addExemplars(m, dp.Exemplars)
+		}
 		ch <- m
 	}
 }

exporters/prometheus/exporter_test.go

@@ -431,6 +431,42 @@ func TestPrometheusExporter(t *testing.T) {
 				counter.Add(ctx, 5, otelmetric.WithAttributeSet(attrs2))
 			},
 		},
+		{
+			name:         "non-monotonic sum does not add exemplars",
+			expectedFile: "testdata/non_monotonic_sum_does_not_add_exemplars.txt",
+			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
+				sc := trace.NewSpanContext(trace.SpanContextConfig{
+					SpanID:     trace.SpanID{0o1},
+					TraceID:    trace.TraceID{0o1},
+					TraceFlags: trace.FlagsSampled,
+				})
+				ctx = trace.ContextWithSpanContext(ctx, sc)
+				opt := otelmetric.WithAttributes(
+					attribute.Key("A").String("B"),
+					attribute.Key("C").String("D"),
+					attribute.Key("E").Bool(true),
+					attribute.Key("F").Int(42),
+				)
+				counter, err := meter.Float64UpDownCounter(
+					"foo",
+					otelmetric.WithDescription("a simple up down counter"),
+					otelmetric.WithUnit("s"),
+				)
+				require.NoError(t, err)
+				counter.Add(ctx, 5, opt)
+				counter.Add(ctx, 10.3, opt)
+				counter.Add(ctx, 9, opt)
+				counter.Add(ctx, -1, opt)
+
+				attrs2 := attribute.NewSet(
+					attribute.Key("A").String("D"),
+					attribute.Key("C").String("B"),
+					attribute.Key("E").Bool(true),
+					attribute.Key("F").Int(42),
+				)
+				counter.Add(ctx, 5, otelmetric.WithAttributeSet(attrs2))
+			},
+		},
 	}
 
 	for _, tc := range testCases {

exporters/prometheus/testdata/non_monotonic_sum_does_not_add_exemplars.txt

@@ -0,0 +1,10 @@
+# HELP foo_seconds a simple up down counter
+# TYPE foo_seconds gauge
+foo_seconds{A="B",C="D",E="true",F="42",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 23.3
+foo_seconds{A="D",C="B",E="true",F="42",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 5
+# HELP otel_scope_info Instrumentation Scope metadata
+# TYPE otel_scope_info gauge
+otel_scope_info{otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 1
+# HELP target_info Target metadata
+# TYPE target_info gauge
+target_info{service_name="prometheus_test",telemetry_sdk_language="go",telemetry_sdk_name="opentelemetry",telemetry_sdk_version="latest"} 1