Track Action Cable Message Size in Prometheus

#296845 (closed) Introduced counts of events emitted by Action Cable.

In order to understand the quantity of data transmitted over WebSocket connections (and in turn provide data for discussions around buffer size in Workhorse), it would be useful to track the size of messages being sent over WebSocket connections.

This will change with the merging of !43333 (closed) as we move from sending a simple ping to GraphQL responses over the wire. It may further evolve as bidirection/collaborative editing is developed and larger transactions are handled.