Add the internal experimental metric feature package

sdk/metric/internal/x/x.go

@@ -0,0 +1,89 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package x contains support for OTel metric SDK experimental features.
+//
+// This package should only be used for features defined in the specification.
+// It should not be used for experiments or new project ideas.
+package x // import "go.opentelemetry.io/otel/sdk/metric/internal/x"
+
+import (
+	"os"
+	"strconv"
+	"strings"
+)
+
+var (
+	// Exemplars is an experimental feature flag that defines if exemplars
+	// should be recorded for metric data-points.
+	//
+	// To enable this feature set the OTEL_GO_X_EXEMPLAR environment variable
+	// to the case-insensitive string value of "true" (i.e. "True" and "TRUE"
+	// will also enable this).
+	Exemplars = newFeature("EXEMPLAR", func(v string) (string, bool) {
+		if strings.ToLower(v) == "true" {
+			return v, true
+		}
+		return "", false
+	})
+
+	// CardinalityLimit is an experimental feature flag that defines if
+	// cardinality limits should be applied to the recorded metric data-points.
+	//
+	// To enable this feature set the OTEL_GO_X_CARDINALITY_LIMIT environment
+	// variable to the integer limit value you want to use.
+	CardinalityLimit = newFeature("CARDINALITY_LIMIT", func(v string) (int, bool) {
+		n, err := strconv.Atoi(v)
+		if err != nil {
+			return 0, false
+		}
+		return n, true
+	})
+)
+
+// Feature is an experimental feature control flag. It provides a uniform way
+// to interact with these feature flags and parse their values.
+type Feature[T any] struct {
+	key   string
+	parse func(v string) (T, bool)
+}
+
+func newFeature[T any](suffix string, parse func(string) (T, bool)) Feature[T] {
+	const envKeyRoot = "OTEL_GO_X_"
+	return Feature[T]{
+		key:   envKeyRoot + suffix,
+		parse: parse,
+	}
+}
+
+// Key returns the environment variable key that needs to be set to enable the
+// feature.
+func (f Feature[T]) Key() string { return f.key }
+
+// Lookup returns the user configured value for the feature and true if the
+// user has enabled the feature. Otherwise, if the feature is not enabled, a
+// zero-value and false are returned.
+func (f Feature[T]) Lookup() (v T, ok bool) {
+	vRaw, present := os.LookupEnv(f.key)
+	if !present {
+		return v, ok
+	}
+	return f.parse(vRaw)
+}
+
+// Enabled returns if the feature is enabled.
+func (f Feature[T]) Enabled() bool {
+	_, ok := f.Lookup()
+	return ok
+}