Add custom Diamond collector for RCStream

This change depends on change Ia9de677c8 in mediawiki/services/rcstream.
It provisions a custom Diamond metrics collector for RCStream which connects to
all configured backends and reports the total count of connected clients and
the maximum queue size.

Change-Id: I37f3f38c9f274f17a0b6e8a430d7a75f8486dc74