address fixme

src/core/xds/grpc/xds_cluster.cc

@@ -53,7 +53,11 @@ std::string XdsClusterResource::ToString() const {
     contents.push_back(absl::StrCat("lrs_load_reporting_server_name=",
                                     lrs_load_reporting_server->server_uri()));
   }
-// FIXME: include lrs_backend_metric_propagation
+  if (lrs_backend_metric_propagation != nullptr) {
+    contents.push_back(
+        absl::StrCat("lrs_backend_metric_propagation=",
+                     lrs_backend_metric_propagation->AsString()));
+  }
   if (!common_tls_context.Empty()) {
     contents.push_back(
         absl::StrCat("common_tls_context=", common_tls_context.ToString()));