member_id is the ID of the member which sent the response.
raft_term is the raft term when the request was applied.
revision is the key-value store revision when the request was applied, and it's unset (so 0) in case of calls not interacting with key-value store. For watch progress responses, the header.revision indicates progress. All future events received in this stream are guaranteed to have a higher revision number than the header.revision number.
cluster_id is the ID of the cluster which sent the response.