Ratelimit: Add default value to dynamic metadata action (#11873)

Modified dynamic_metadata action to now accept an optional default
value for instances where no value is queried from the dynamic metadata.

Signed-off-by: Clara Andrew-Wani <candrewwani@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3afa3b50eacfa39fa5b3518b05b03689dc56ef42

envoy/config/route/v3/route_components.proto

@@ -1490,6 +1490,10 @@ message RateLimit {
       // Metadata struct that defines the key and path to retrieve the string value. A match will
       // only happen if the value in the dynamic metadata is of type string.
       type.metadata.v3.MetadataKey metadata_key = 2 [(validate.rules).message = {required: true}];
+
+      // An optional value to use if *metadata_key* is empty. If not set and
+      // no value is present under the metadata_key then no descriptor is generated.
+      string default_value = 3;
     }
 
     oneof action_specifier {

envoy/config/route/v4alpha/route_components.proto

@@ -1472,6 +1472,10 @@ message RateLimit {
       // Metadata struct that defines the key and path to retrieve the string value. A match will
       // only happen if the value in the dynamic metadata is of type string.
       type.metadata.v3.MetadataKey metadata_key = 2 [(validate.rules).message = {required: true}];
+
+      // An optional value to use if *metadata_key* is empty. If not set and
+      // no value is present under the metadata_key then no descriptor is generated.
+      string default_value = 3;
     }
 
     oneof action_specifier {