Drop overload runtime key (#33737)

Adding runtime key to protect drop overload feature.

The runtime key "load_balancing_policy.drop_overload_limit" can be configured with an integer 0 to 100. 0 means 0%. 100 means 100%. So, when there is an EDS update with drop_overloads configuration, if this runtime key is enabled, Envoy will pick up the smaller one between these two to perform the drops.

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

Mirrored from https://github.com/envoyproxy/envoy @ ef8a34d89f85f434e6df562c742b63a359d0ceb4

envoy/config/endpoint/v3/endpoint.proto

@@ -77,6 +77,12 @@ message ClusterLoadAssignment {
     //
     // 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.
+    //
+    // In Envoy, this ``drop_overloads`` config can be overridden by a runtime key
+    // "load_balancing_policy.drop_overload_limit" setting. This runtime key can be set to
+    // any integer number between 0 and 100. 0 means drop 0%. 100 means drop 100%.
+    // When both ``drop_overloads`` config and "load_balancing_policy.drop_overload_limit"
+    // setting are in place, the min of these two wins.
     repeated DropOverload drop_overloads = 2;
 
     // Priority levels and localities are considered overprovisioned with this