prometheus: add WithNamespace option to prefix metrics (#3970)

* prometheus: add WithNamespace option to prefix metrics

* sanitize namespace, add config test

* Update CHANGELOG.md

Co-authored-by: Robert Pająk <pellared@hotmail.com>

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - The `go.opentelemetry.io/otel/metric/embedded` package. (#3916)
 - The `Version` function to `go.opentelemetry.io/otel/sdk` to return the SDK version. (#3949)
+- Add a `WithNamespace` option to `go.opentelemetry.io/otel/exporters/prometheus` to allow users to prefix metrics with a namespace. (#3970)
 
 ### Changed
 

exporters/prometheus/config.go

@@ -15,6 +15,8 @@
 package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 
 import (
+	"strings"
+
 	"github.com/prometheus/client_golang/prometheus"
 
 	"go.opentelemetry.io/otel/sdk/metric"
@@ -27,6 +29,7 @@ type config struct {
 	withoutUnits      bool
 	aggregation       metric.AggregationSelector
 	disableScopeInfo  bool
+	namespace         string
 }
 
 // newConfig creates a validated config configured with options.
@@ -116,3 +119,20 @@ func WithoutScopeInfo() Option {
 		return cfg
 	})
 }
+
+// WithNamespace configures the Exporter to prefix metric with the given namespace.
+// Metadata metrics such as target_info and otel_scope_info are not prefixed since these
+// have special behavior based on their name.
+func WithNamespace(ns string) Option {
+	return optionFunc(func(cfg config) config {
+		ns = sanitizeName(ns)
+		if !strings.HasSuffix(ns, "_") {
+			// namespace and metric names should be separated with an underscore,
+			// adds a trailing underscore if there is not one already.
+			ns = ns + "_"
+		}
+
+		cfg.namespace = ns
+		return cfg
+	})
+}

exporters/prometheus/config_test.go

@@ -99,6 +99,36 @@ func TestNewConfig(t *testing.T) {
 				withoutUnits: true,
 			},
 		},
+		{
+			name: "with namespace",
+			options: []Option{
+				WithNamespace("test"),
+			},
+			wantConfig: config{
+				registerer: prometheus.DefaultRegisterer,
+				namespace:  "test_",
+			},
+		},
+		{
+			name: "with namespace with trailing underscore",
+			options: []Option{
+				WithNamespace("test_"),
+			},
+			wantConfig: config{
+				registerer: prometheus.DefaultRegisterer,
+				namespace:  "test_",
+			},
+		},
+		{
+			name: "with unsanitized namespace",
+			options: []Option{
+				WithNamespace("test/"),
+			},
+			wantConfig: config{
+				registerer: prometheus.DefaultRegisterer,
+				namespace:  "test_",
+			},
+		},
 	}
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {

exporters/prometheus/exporter.go

@@ -66,6 +66,7 @@ type collector struct {
 	createTargetInfoOnce sync.Once
 	scopeInfos           map[instrumentation.Scope]prometheus.Metric
 	metricFamilies       map[string]*dto.MetricFamily
+	namespace            string
 }
 
 // prometheus counters MUST have a _total suffix:
@@ -88,6 +89,7 @@ func New(opts ...Option) (*Exporter, error) {
 		disableScopeInfo:  cfg.disableScopeInfo,
 		scopeInfos:        make(map[instrumentation.Scope]prometheus.Metric),
 		metricFamilies:    make(map[string]*dto.MetricFamily),
+		namespace:         cfg.namespace,
 	}
 
 	if err := cfg.registerer.Register(collector); err != nil {
@@ -316,9 +318,12 @@ var unitSuffixes = map[string]string{
 	"ms": "_milliseconds",
 }
 
-// getName returns the sanitized name, including unit suffix.
+// getName returns the sanitized name, prefixed with the namespace and suffixed with unit.
 func (c *collector) getName(m metricdata.Metrics) string {
 	name := sanitizeName(m.Name)
+	if c.namespace != "" {
+		name = c.namespace + name
+	}
 	if c.withoutUnits {
 		return name
 	}

exporters/prometheus/exporter_test.go

@@ -258,6 +258,26 @@ func TestPrometheusExporter(t *testing.T) {
 				counter.Add(ctx, 1, attrs...)
 			},
 		},
+		{
+			name:         "with namespace",
+			expectedFile: "testdata/with_namespace.txt",
+			options: []Option{
+				WithNamespace("test"),
+			},
+			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
+				attrs := []attribute.KeyValue{
+					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", instrument.WithDescription("a simple counter"))
+				require.NoError(t, err)
+				counter.Add(ctx, 5, attrs...)
+				counter.Add(ctx, 10.3, attrs...)
+				counter.Add(ctx, 9, attrs...)
+			},
+		},
 	}
 
 	for _, tc := range testCases {

exporters/prometheus/testdata/with_namespace.txt

@@ -0,0 +1,9 @@
+# HELP test_foo_total a simple counter
+# TYPE test_foo_total counter
+test_foo_total{A="B",C="D",E="true",F="42",otel_scope_name="testmeter",otel_scope_version="v0.1.0"} 24.3
+# 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