api: use application_utilization in WRR (#27807)

* api: use application_utilization in WRR

Signed-off-by: Yousuk Seung <ysseung@google.com>

* Comment addressed

Signed-off-by: Yousuk Seung <ysseung@google.com>

---------

Signed-off-by: Yousuk Seung <ysseung@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ c5b204ce93f2557c9f1e5868ace160c5f703f89e

envoy/extensions/load_balancing_policies/client_side_weighted_round_robin/v3/client_side_weighted_round_robin.proto

@@ -22,14 +22,15 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // This policy differs from the built-in ROUND_ROBIN policy in terms of
 // how the endpoint weights are determined. In the ROUND_ROBIN policy,
 // the endpoint weights are sent by the control plane via EDS. However,
-// in this policy, the endpoint weights are instead determined via
-// qps (queries per second), eps (errors per second), and CPU utilization
-// metrics sent by the endpoint using the Open Request Cost Aggregation (ORCA)
-// protocol. A query counts towards qps when successful, otherwise towards both
-// qps and eps. What counts as an error is up to the endpoint to define.
-// A config parameter error_utilization_penalty controls the penalty to adjust
-// endpoint weights using eps and qps. The weight of a given endpoint is
-// computed as: qps / (cpu_utilization + eps/qps * error_utilization_penalty)
+// in this policy, the endpoint weights are instead determined via qps (queries
+// per second), eps (errors per second), and utilization metrics sent by the
+// endpoint using the Open Request Cost Aggregation (ORCA) protocol. Utilization
+// is determined by using the ORCA application_utilization field, if set, or
+// else falling back to the cpu_utilization field. All queries count toward qps,
+// regardless of result. Only failed queries count toward eps. A config
+// parameter error_utilization_penalty controls the penalty to adjust endpoint
+// weights using eps and qps. The weight of a given endpoint is computed as:
+//   qps / (utilization + eps/qps * error_utilization_penalty)
 //
 // See the :ref:`load balancing architecture overview<arch_overview_load_balancing_types>` for more information.
 //