runsc: Add experiment metadata metric label.

This is currently unused, but may be used to contain the name of an experiment
that is enabled on `runsc` in the future.

Defining the label now means less chance of breakage and less hassle relative
to introducing it later.

PiperOrigin-RevId: 519855249

runsc/config/config.go

@@ -405,6 +405,9 @@ func (c *Config) MetricMetadata() map[string]string {
 		"fsmode":   fsMode,
 		"cpuarch":  runtime.GOARCH,
 		"go":       runtime.Version(),
+		// The "experiment" label is currently unused, but may be used to contain
+		// extra information about e.g. an experiment that may be enabled.
+		"experiment": "",
 	}
 }