route: add stats prefix for route (#21302)

Commit Message: add stats prefix for route
Additional Description: Adds API for stats generation support per route.
Risk Level: Low
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:

API for #3351

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7b937e8cd5796cf19d14f20abcb6be81f149c68b

envoy/config/route/v3/route_components.proto

@@ -219,7 +219,7 @@ message FilterAction {
 //
 //   Envoy supports routing on HTTP method via :ref:`header matching
 //   <envoy_v3_api_msg_config.route.v3.HeaderMatcher>`.
-// [#next-free-field: 19]
+// [#next-free-field: 20]
 message Route {
   option (udpa.annotations.versioning).previous_message_type = "envoy.api.v2.route.Route";
 
@@ -317,6 +317,21 @@ message Route {
   // If set, the bytes actually buffered will be the minimum value of this and the
   // listener per_connection_buffer_limit_bytes.
   google.protobuf.UInt32Value per_request_buffer_limit_bytes = 16;
+
+  // The human readable prefix to use when emitting statistics for this endpoint.
+  // The statistics are rooted at vhost.<virtual host name>.route.<stat_prefix>.
+  // This should be set for highly critical
+  // endpoints that one wishes to get “per-route” statistics on.
+  // If not set, endpoint statistics are not generated.
+  //
+  // The emitted statistics are the same as those documented for :ref:`virtual clusters <config_http_filters_router_vcluster_stats>`.
+  //
+  // .. warning::
+  //
+  //    We do not recommend setting up a stat prefix for
+  //    every application endpoint. This is both not easily maintainable and
+  //    statistics use a non-trivial amount of memory(approximately 1KiB per route).
+  string stat_prefix = 19;
 }
 
 // Compared to the :ref:`cluster <envoy_v3_api_field_config.route.v3.RouteAction.cluster>` field that specifies a