Enhance comment about MonotonicTime

api/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.

source/common/upstream/cluster_manager_impl.h

@@ -419,9 +419,13 @@ class ClusterManagerImpl : public ClusterManager, Logger::Loggable<Logger::Id::u
     // This is default constructed to the clock's epoch:
     // https://en.cppreference.com/w/cpp/chrono/time_point/time_point
     //
-    // This will usually be the computer's boot time, which means that given a not very large
-    // `Cluster.CommonLbConfig.update_merge_window`, the first update will trigger immediately
-    // (the expected behavior).
+    // Depending on your execution environment this value can be different.
+    // When running as host process: This will usually be the computer's boot time, which means that
+    // given a not very large `Cluster.CommonLbConfig.update_merge_window`, the first update will
+    // trigger immediately (the expected behavior). When running in some sandboxed environment this
+    // value can be set to the start time of the sandbox, which means that the delta calculated
+    // between now and the start time may fall within the
+    // `Cluster.CommonLbConfig.update_merge_window`, with the side effect to delay the first update.
     MonotonicTime last_updated_;
   };