Add the sdk/internal/x package

Add the `x` package to handle experimental feature flagging within the
go.opentelemetry.io/otel/sdk module.

sdk/internal/x/README.md

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+# Experimental Features
+
+The SDK contains features that have not yet stabilized in the OpenTelemetry specification.
+These features are added to the OpenTelemetry Go SDK prior to stabilization in the specification so that users can start experimenting with them and provide feedback.
+
+These feature may change in backwards incompatible ways as feedback is applied.
+See the [Compatibility and Stability](#compatibility-and-stability) section for more information.
+
+## Features
+
+- [Resource](#resource)
+
+### Resource
+
+[OpenTelemetry resource semantic conventions] include many attribute definitions that are defined as experimental.
+To have experimental semantic conventions be added by [resource detectors] set the `OTEL_GO_X_RESOURCE` environment variable.
+The value set must be the case-insensitive string of `"true"` to enable the feature.
+All other values are ignored.
+
+<!-- TODO: document what attributes are added by which detector -->
+
+[OpenTelemetry resource semantic conventions]: https://opentelemetry.io/docs/specs/semconv/resource/
+[resource detectors]: https://pkg.go.dev/go.opentelemetry.io/otel/sdk/resource#Detector
+
+#### Examples
+
+Enable experimental resource semantic conventions.
+
+```console
+export OTEL_GO_X_RESOURCE=true
+```
+
+Disable experimental resource semantic conventions.
+
+```console
+unset OTEL_GO_X_RESOURCE
+```
+
+## Compatibility and Stability
+
+Experimental features do not fall within the scope of the OpenTelemetry Go versioning and stability [policy](../../../../VERSIONING.md).
+These features may be removed or modified in successive version releases, including patch versions.
+
+When an experimental feature is promoted to a stable feature, a migration path will be included in the changelog entry of the release.
+There is no guarantee that any environment variable feature flags that enabled the experimental feature will be supported by the stable version.
+If they are supported, they may be accompanied with a deprecation notice stating a timeline for the removal of that support.

sdk/internal/x/x.go

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+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package x contains support for OTel 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/internal/x"
+
+import (
+	"os"
+	"strings"
+)
+
+// Resource is an experimental feature flag that defines if resource detectors
+// should be included experimental semantic conventions.
+//
+// To enable this feature set the OTEL_GO_X_RESOURCE environment variable
+// to the case-insensitive string value of "true" (i.e. "True" and "TRUE"
+// will also enable this).
+var Resource = newFeature("RESOURCE", func(v string) (string, bool) {
+	if strings.ToLower(v) == "true" {
+		return v, true
+	}
+	return "", false
+})
+
+// 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) {
+	// https://github.com/open-telemetry/opentelemetry-specification/blob/62effed618589a0bec416a87e559c0a9d96289bb/specification/configuration/sdk-environment-variables.md#parsing-empty-value
+	//
+	// > The SDK MUST interpret an empty value of an environment variable the
+	// > same way as when the variable is unset.
+	vRaw := os.Getenv(f.key)
+	if vRaw == "" {
+		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
+}

sdk/internal/x/x_test.go

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+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package x
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestExemplars(t *testing.T) {
+	const key = "OTEL_GO_X_RESOURCE"
+	require.Equal(t, key, Resource.Key())
+
+	t.Run("true", run(setenv(key, "true"), assertEnabled(Resource, "true")))
+	t.Run("True", run(setenv(key, "True"), assertEnabled(Resource, "True")))
+	t.Run("TRUE", run(setenv(key, "TRUE"), assertEnabled(Resource, "TRUE")))
+	t.Run("false", run(setenv(key, "false"), assertDisabled(Resource)))
+	t.Run("1", run(setenv(key, "1"), assertDisabled(Resource)))
+	t.Run("empty", run(assertDisabled(Resource)))
+}
+
+func run(steps ...func(*testing.T)) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+		for _, step := range steps {
+			step(t)
+		}
+	}
+}
+
+func setenv(k, v string) func(t *testing.T) {
+	return func(t *testing.T) { t.Setenv(k, v) }
+}
+
+func assertEnabled[T any](f Feature[T], want T) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+		assert.True(t, f.Enabled(), "not enabled")
+
+		v, ok := f.Lookup()
+		assert.True(t, ok, "Lookup state")
+		assert.Equal(t, want, v, "Lookup value")
+	}
+}
+
+func assertDisabled[T any](f Feature[T]) func(*testing.T) {
+	var zero T
+	return func(t *testing.T) {
+		t.Helper()
+
+		assert.False(t, f.Enabled(), "enabled")
+
+		v, ok := f.Lookup()
+		assert.False(t, ok, "Lookup state")
+		assert.Equal(t, zero, v, "Lookup value")
+	}
+}