metadata property
Service-specific metadata associated with the operation.
It typically contains progress information and common metadata such as create time. Some services might not provide such metadata. Any method that returns a long-running operation should document the metadata type, if any.
The values for Object must be JSON objects. It can consist of num,
String, bool and null as well as Map and List values.
Implementation
core.Map<core.String, core.Object?>? metadata;