Cannot run more than one wal.LiveReader with metrics

[csmarchbanks]

#619 changed the LiveReader to take a prometheus registerer instead of always using the default registerer. However, now if you try to instantiate more than one LiveReader with the same registerer it will panic.

You can pass nil as the registerer to avoid the panic but that means that metric will no longer show up.

cc: @brancz since I think you are doing the Prometheus release soon. This was breaking Prometheus when I tried to update prometheus/prometheus#5592 in preparation for 2.11.