Spec retry_after_ms on ratelimit errors

api/client-server/definitions/errors/rate_limited.yaml

@@ -23,4 +23,10 @@ properties:
     type: string
     description: A human-readable error message.
     example: Too many requests
+  retry_after_ms:
+    type: integer
+    description: |-
+      The amount of time in milliseconds the client should wait
+      before trying the request again.
+    example: 2000
 required: ["errcode"]