ddtrace/tracer: clarify the configuration of dogstatsd including UDS. (…

…#1274)

ddtrace/tracer/option.go

@@ -637,9 +637,12 @@ func WithRuntimeMetrics() StartOption {
 	}
 }
 
-// WithDogstatsdAddress specifies the address to connect to for sending metrics
-// to the Datadog Agent. If not set, it defaults to "localhost:8125" or to the
-// combination of the environment variables DD_AGENT_HOST and DD_DOGSTATSD_PORT.
+// WithDogstatsdAddress specifies the address to connect to for sending metrics to the Datadog
+// Agent. It should be a "host:port" string, or the path to a unix domain socket.If not set, it
+// attempts to determine the address of the statsd service according to the following rules:
+//   1. Look for /var/run/datadog/dsd.socket and use it if present. IF NOT, continue to #2.
+//   2. The host is determined by DD_AGENT_HOST, and defaults to "localhost"
+//   3. The port is retrieved from the agent. If not present, it is determined by DD_DOGSTATSD_PORT, and defaults to 8125
 // This option is in effect when WithRuntimeMetrics is enabled.
 func WithDogstatsdAddress(addr string) StartOption {
 	return func(cfg *config) {