Set zipkin_service_name to LocalInfo::clusterName(). (#7354)

Signed-off-by: Emil Mikulic <g-easy@users.noreply.github.com>

Mirrored from https://github.com/envoyproxy/envoy @ ea3ebca3b6d84a8b29c35ca03fa3666a0f4951c9

envoy/config/trace/v2/trace.proto

@@ -130,8 +130,7 @@ message OpenCensusConfig {
   // The URL to Zipkin, e.g. "http://127.0.0.1:9411/api/v2/spans"
   string zipkin_url = 6;
 
-  // The Zipkin service name.
-  string zipkin_service_name = 7;
+  reserved 7; // Formerly zipkin_service_name.
 
   enum TraceContext {
     // No-op default, no trace context is utilized.