MB-9712: Add the infrastructure to add metadata to UPR

Note that currently no metadata is being sent

Change-Id: I4f12cdf4070a85624e9e86237208615ceaf288f1
Reviewed-on: http://review.couchbase.org/31018
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>