API: Add Overload Manager ResetReason and ResponseFlag. (#17044)

Signed-off-by: Kevin Baichoo <kbaichoo@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 1aa65ac88e9888127fe795e3ecb4c1bbc36534bf

envoy/config/accesslog/v3/accesslog.proto

@@ -246,6 +246,7 @@ message ResponseFlagFilter {
         in: "DT"
         in: "UPE"
         in: "NC"
+        in: "OM"
       }
     }
   }];

envoy/config/accesslog/v4alpha/accesslog.proto

@@ -245,6 +245,7 @@ message ResponseFlagFilter {
         in: "DT"
         in: "UPE"
         in: "NC"
+        in: "OM"
       }
     }
   }];

envoy/data/accesslog/v3/accesslog.proto

@@ -186,7 +186,7 @@ message AccessLogCommon {
 }
 
 // Flags indicating occurrences during request/response processing.
-// [#next-free-field: 26]
+// [#next-free-field: 27]
 message ResponseFlags {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.data.accesslog.v2.ResponseFlags";
@@ -281,6 +281,9 @@ message ResponseFlags {
 
   // Indicates no cluster was found for the request.
   bool no_cluster_found = 25;
+
+  // Indicates overload manager terminated the request.
+  bool overload_manager = 26;
 }
 
 // Properties of a negotiated TLS connection.