stats: DogStatsd sink supports custom metric name prefix (#4506)

ref: https://github.com/envoyproxy/envoy/issues/4390

Signed-off-by: Taiki Ono <taiki-ono@cookpad.com>

Mirrored from https://github.com/envoyproxy/envoy @ bdb2859b044e267bb5c6a76953e8c96b813258da

envoy/config/metrics/v2/stats.proto

@@ -200,6 +200,10 @@ message DogStatsdSink {
   }
 
   reserved 2;
+
+  // Optional custom metric name prefix. See :ref:`StatsdSink's prefix field
+  // <envoy_api_field_config.metrics.v2.StatsdSink.prefix>` for more details.
+  string prefix = 3;
 }
 
 // Stats configuration proto schema for built-in *envoy.stat_sinks.hystrix* sink.