Turn on cluster update merging by default (#5009)

We've been using this in production for over 3 months now and it's
been very useful to prevent CPU spikes when we get a stream of
updates.

This enables update merging every 1s.

Fixes #4018.

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

Mirrored from https://github.com/envoyproxy/envoy @ fad993e5aed40fba95897e9017afd19bdf170ed0

envoy/api/v2/cds.proto

@@ -472,6 +472,9 @@ message Cluster {
     // 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).
     //
+    // If this is not set, we default to a merge window of 1000ms. To disable it, set the merge
+    // window to 0.
+    //
     // 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.