stats: optional per-endpoint metrics (#29709)

Signed-off-by: Greg Greenway <ggreenway@apple.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7fcadfcd0b055cd7dbaf25d1b88618d80a8116a9

envoy/config/cluster/v3/cluster.proto

@@ -1257,4 +1257,19 @@ message TrackClusterStats {
   // <config_cluster_manager_cluster_stats_request_response_sizes>`  tracking header and body sizes
   // of requests and responses will be published.
   bool request_response_sizes = 2;
+
+  // If true, some stats will be emitted per-endpoint, similar to the stats in admin ``/clusters``
+  // output.
+  //
+  // This does not currently output correct stats during a hot-restart.
+  //
+  // This is not currently implemented by all stat sinks.
+  //
+  // These stats do not honor filtering or tag extraction rules in :ref:`StatsConfig
+  // <envoy_v3_api_msg_config.metrics.v3.StatsConfig>` (but fixed-value tags are supported). Admin
+  // endpoint filtering is supported.
+  //
+  // This may not be used at the same time as
+  // :ref:`load_stats_config <envoy_v3_api_field_config.bootstrap.v3.ClusterManager.load_stats_config>`.
+  bool per_endpoint_stats = 3;
 }