make the path of PrometheusMetricsService be different from default /metrics

src/brpc/builtin/prometheus_metrics_service.h

@@ -22,7 +22,7 @@
 
 namespace brpc {
 
-class PrometheusMetricsService : public metrics {
+class PrometheusMetricsService : public brpc_vars_metrics {
 public:
     PrometheusMetricsService(Server* server)
         : _server(server) {}

src/brpc/builtin_service.proto

@@ -97,7 +97,7 @@ service sockets {
     rpc default_method(SocketsRequest) returns (SocketsResponse);
 }
 
-service metrics {
+service brpc_vars_metrics {
     rpc default_method(MetricsRequest) returns (MetricsResponse);
 }