proto: add a new field called append_action in the HeaderValueOption

api/envoy/config/core/v3/base.proto

@@ -320,12 +320,33 @@ message HeaderValueOption {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.api.v2.core.HeaderValueOption";
 
+  // Describes the supported actions types for header append action.
+  enum HeaderAppendAction {
+    // This action will append the specified value to the existing values if the header
+    // already exists. If the header doesn't exist then this will add the header with
+    // specified key and value.
+    APPEND_IF_EXISTS_OR_ADD = 0;
+
+    // This action will add the header if it doesn't already exist. If the header
+    // already exists then this will be a no-op.
+    ADD_IF_ABSENT = 1;
+
+    // This action will overwrite the specified value by discarding any existing values if
+    // the header already exists. If the header doesn't exist then this will add the header
+    // with specified key and value.
+    OVERWRITE_IF_EXISTS_OR_ADD = 2;
+  }
+
   // Header name/value pair that this option applies to.
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
   // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
+
+  // [#not-implemented-hide:] Describes the action taken to append/overwrite the given value for an existing header
+  // or to only add this header if it's absent. Value defaults to :ref:`APPEND_IF_EXISTS_OR_ADD<envoy_v3_api_enum_value_config.core.v3.HeaderValueOption.HeaderAppendAction.APPEND_IF_EXISTS_OR_ADD>`.
+  HeaderAppendAction append_action = 3 [(validate.rules).enum = {defined_only: true}];
 }
 
 // Wrapper for a set of headers.