Add option for merging cluster updates (#3941)

This change introduces a new configuration parameter for clusters,
`time_between_updates`.

If this is set, all cluster updates — membership changes, metadata
changes on endpoints, or healtcheck state changes — that happen
within a `time_between_updates` duration will be merged and delivered
at once when the duration ends.

This is useful for big clusters (> 1k endpoints) using the subset LB.

Partially addresses https://github.com/envoyproxy/envoy/issues/3929.

Signed-off-by: Raul Gutierrez Segales <rgs@pinterest.com>

Mirrored from https://github.com/envoyproxy/envoy @ 62441f9fe2933d4846cfc3d166e52a9178be755f

envoy/api/v2/cds.proto

@@ -437,6 +437,17 @@ message Cluster {
       ZoneAwareLbConfig zone_aware_lb_config = 2;
       LocalityWeightedLbConfig locality_weighted_lb_config = 3;
     }
+    // If set, all health check/weight/metadata updates that happen within this duration will be
+    // merged and delivered in one shot when the duration expires. The start of the duration is when
+    // the first update happens. This is useful for big clusters, with potentially noisy deploys
+    // that might trigger excessive CPU usage due to a constant stream of healthcheck state changes
+    // or metadata updates. By default, this is not configured and updates apply immediately. Also,
+    // the first set of updates to be seen apply immediately as well (e.g.: a new cluster).
+    //
+    // Note: merging does not apply to cluster membership changes (e.g.: adds/removes); this is
+    // because merging those updates isn't currently safe. See
+    // https://github.com/envoyproxy/envoy/pull/3941.
+    google.protobuf.Duration update_merge_window = 4;
   }
 
   // Common configuration for all load balancer implementations.