A58 update: Prefer application_utilization to cpu_utilization

A58-client-side-weighted-round-robin-lb-policy.md

@@ -125,20 +125,20 @@ In all languages, when a subchannel state is updated to `IDLE`, the LB policy im
 
 ### Subchannel Weights
 
-To enforce `blackout_period` and `weight_expiration_period`, the LB policy tracks two timestamps along with the weight. `last_updated` tracks the last time a report with non-zero `cpu_utilization` and `qps` was received. `non_empty_since` tracks when the first non-zero load report was received, and it is reset when a subchannel comes back to `READY` or the weight expires so that it can be updated with the next non-zero load report. The blackout and expiration are enforced when the weight value is looked up.
+To enforce `blackout_period` and `weight_expiration_period`, the LB policy tracks two timestamps along with the weight. `last_updated` tracks the last time a report with non-zero `application_utilization` and `qps` was received. `non_empty_since` tracks when the first non-zero load report was received, and it is reset when a subchannel comes back to `READY` or the weight expires so that it can be updated with the next non-zero load report. The blackout and expiration are enforced when the weight value is looked up.
 
-The weight of a backend is calculated using `qps` (queries per second), `eps` (errors per second) and `cpu_utilization` reported by the backend. `eps` is only used when `error_utilization_penalty` is set to non-zero and applies a penalty based on the error rate calculated as `eps/qps`. The formula used is as follows:
+The weight of a backend is calculated using `qps` (queries per second), `eps` (errors per second) and `application_utilization` reported by the backend. `application_utilization` is a metric defined by the server application. This value is expected to range between [0.0, 1.0] normally but may exceed 1.0 when the usage exceeds the reporter dependent soft limits. The policy defaults to `cpu_utilization` when `application_utilization` is not defined in the load report. `eps` is only used when `error_utilization_penalty` is set to non-zero and applies a penalty based on the error rate calculated as `eps/qps`. The formula used is as follows:
 
-$$weight = \dfrac{qps}{cpu\\_utilization + \dfrac{eps}{qps} * error\\_utilization\\_penalty}$$
+$$weight = \dfrac{qps}{application\\_utilization + \dfrac{eps}{qps} * error\\_utilization\\_penalty}$$
 
 In Java and Go, the weight will be stored in a wrapped subchannel. However, in C++, that approach won't work, because we create a new subchannel object for each address whenever we get a new address list, and we don't want to lose existing weight information when that happens. So instead, we store the weights in a map keyed by address, and each subchannel list will take its own ref to the entry in the map for each subchannel in the list.
 
 ```
-function UpdateWeight(qps, eps, cpu_utilization) {
-  if (cpu_utilization > 0 && qps > 0)
-    cpu_utilization += eps / qps * config.error_utilization_penalty;
+function UpdateWeight(qps, eps, application_utilization) {
+  if (application_utilization > 0 && qps > 0)
+    application_utilization += eps / qps * config.error_utilization_penalty;
   var new_weight =
-     cpu_utilization == 0 ? 0 : qps / cpu_utilization;
+     application_utilization == 0 ? 0 : qps / application_utilization;
   if (new_weight == 0) return;
   if (non_empty_since_ == infy) non_empty_since_ = now();
   last_updated_ = now();