Support drop_overload API (#30827)

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Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ae8a5298a73d61d945d947800080fbb4f221dd59

envoy/config/endpoint/v3/endpoint.proto

@@ -40,7 +40,6 @@ message ClusterLoadAssignment {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.api.v2.ClusterLoadAssignment.Policy";
 
-    // [#not-implemented-hide:]
     message DropOverload {
       option (udpa.annotations.versioning).previous_message_type =
           "envoy.api.v2.ClusterLoadAssignment.Policy.DropOverload";
@@ -75,7 +74,9 @@ message ClusterLoadAssignment {
     //    "throttle"_drop = 60%
     //    "lb"_drop = 20%  // 50% of the remaining 'actual' load, which is 40%.
     //    actual_outgoing_load = 20% // remaining after applying all categories.
-    // [#not-implemented-hide:]
+    //
+    // Envoy supports only one element and will NACK if more than one element is present.
+    // Other xDS-capable data planes will not necessarily have this limitation.
     repeated DropOverload drop_overloads = 2;
 
     // Priority levels and localities are considered overprovisioned with this