Enhance comment about MonotonicTime (#8011)

Depending on the execution environment in which envoy is being run, it
is possible that some of the assumption on the clock are maybe not
holding as previously commented. With some sandboxing technologies the
clock does not reference the machine boot time but the sandbox boot
time. This invalidates the assumtpion that the first update in the
cluster_manager will most likely fall out of the windows and ends up
showing a non intuitive behavior difficult to catch.
This PR simply adds a comment that will allow the reader to consider
this option while reading to the code.

Signed-off-by: Flavio Crisciani <f.crisciani@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 640b5a436d2ce8e637d28225d5b4f0aae307dede

envoy/api/v2/cds.proto

@@ -553,7 +553,8 @@ message Cluster {
     // 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. The first set of updates to be seen apply immediately (e.g.: a new
-    // cluster).
+    // cluster). Please always keep in mind that the use of sandbox technologies may change this
+    // behavior.
     //
     // If this is not set, we default to a merge window of 1000ms. To disable it, set the merge
     // window to 0.