Handle custom metric suffix in exporter/prometheus to match otel-contrib (#6839)

Related to
https://github.com/open-telemetry/opentelemetry-go/issues/6704#issuecomment-2916325796

---------

Signed-off-by: Alexandre Lamarre <alex7285@gmail.com>
Co-authored-by: David Ashpole <dashpole@google.com>

CHANGELOG.md

@@ -19,6 +19,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add metric's scope attributes as `otel_scope_[attribute]` labels in `go.opentelemetry.io/otel/exporters/prometheus`. (#5947)
 - Add `EventName` to `EnabledParameters` in `go.opentelemetry.io/otel/log`. (#6825)
 - Add `EventName` to `EnabledParameters` in `go.opentelemetry.io/otel/sdk/log`. (#6825)
+- Changed handling of `go.opentelemetry.io/otel/exporters/prometheus` metric renaming to add unit suffixes when it doesn't match one of the pre-defined values in the unit suffix map. (#6839)
 
 ### Removed
 

exporters/prometheus/exporter.go

@@ -425,6 +425,13 @@ func createInfoMetric(name, description string, res *resource.Resource) (prometh
 	return prometheus.NewConstMetric(desc, prometheus.GaugeValue, float64(1), values...)
 }
 
+func unitMapGetOrDefault(unit string) string {
+	if promUnit, ok := unitSuffixes[unit]; ok {
+		return promUnit
+	}
+	return unit
+}
+
 var unitSuffixes = map[string]string{
 	// Time
 	"d":   "days",
@@ -483,7 +490,7 @@ func (c *collector) getName(m metricdata.Metrics, typ *dto.MetricType) string {
 	if c.namespace != "" {
 		name = c.namespace + name
 	}
-	if suffix, ok := unitSuffixes[m.Unit]; ok && !c.withoutUnits && !strings.HasSuffix(name, suffix) {
+	if suffix := unitMapGetOrDefault(m.Unit); suffix != "" && !c.withoutUnits && !strings.HasSuffix(name, suffix) {
 		name += "_" + suffix
 	}
 	if addCounterSuffix {

exporters/prometheus/exporter_test.go

@@ -95,6 +95,35 @@ func TestPrometheusExporter(t *testing.T) {
 				counter.Add(ctx, 5, otelmetric.WithAttributeSet(attrs2))
 			},
 		},
+		{
+			name:         "counter with custom unit not tracked by ucum standards",
+			expectedFile: "testdata/counter_with_custom_unit_suffix.txt",
+			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
+				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.Float64Counter(
+					"foo",
+					otelmetric.WithDescription("a simple counter"),
+					otelmetric.WithUnit("madeup"),
+				)
+				require.NoError(t, err)
+				counter.Add(ctx, 5, opt)
+				counter.Add(ctx, 10.3, opt)
+				counter.Add(ctx, 9, 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))
+			},
+		},
 		{
 			name:         "counter that already has a total suffix",
 			expectedFile: "testdata/counter.txt",

exporters/prometheus/testdata/counter_with_custom_unit_suffix.txt

@@ -0,0 +1,7 @@
+# HELP "foo_madeup_total" a simple counter
+# TYPE "foo_madeup_total" counter
+{"foo_madeup_total",A="B",C="D",E="true",F="42",otel_scope_fizz="buzz",otel_scope_name="testmeter",otel_scope_schema_url="",otel_scope_version="v0.1.0"} 24.3
+{"foo_madeup_total",A="D",C="B",E="true",F="42",otel_scope_fizz="buzz",otel_scope_name="testmeter",otel_scope_schema_url="",otel_scope_version="v0.1.0"} 5
+# 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