add OTEL_GO_X_SELF_OBSERVABILITY feature gate

Author: dashpole

sdk/internal/x/README.md

@@ -22,6 +22,13 @@ All other values are ignored.
 [OpenTelemetry resource semantic conventions]: https://opentelemetry.io/docs/specs/semconv/resource/
 [resource detectors]: https://pkg.go.dev/go.opentelemetry.io/otel/sdk/resource#Detector
 
+### SDK Self-Observability
+
+To enable experimental metric and trace instrumentation in SDKs, set the `OTEL_GO_X_SELF_OBSERVABILITY` environment variable.
+If enabled, this instrumentation uses the global `TracerProvider` and `MeterProvider`.
+The value set must be the case-insensitive string of `"true"` to enable the feature.
+All other values are ignored.
+
 #### Examples
 
 Enable experimental resource semantic conventions.

sdk/internal/x/x.go

@@ -25,6 +25,19 @@ var Resource = newFeature("RESOURCE", func(v string) (string, bool) {
 	return "", false
 })
 
+// SelfObservability is an experimental feature flag that determines if SDK
+// self-observability metrics are enabled.
+//
+// To enable this feature set the OTEL_GO_X_SELF_OBSERVABILITY environment variable
+// to the case-insensitive string value of "true" (i.e. "True" and "TRUE"
+// will also enable this).
+var SelfObservability = newFeature("SELF_OBSERVABILITY", 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 {