Make metrics exported by the Prometheus server itself more consistent.

- Always spell out the time unit (e.g. milliseconds instead of ms).

- Remove "_total" from the names of metrics that are not counters.

- Make use of the "Namespace" and "Subsystem" fields in the options.

- Removed the "capacity" facet from all metrics about channels/queues.
  These are all fixed via command line flags and will never change
  during the runtime of a process. Also, they should not be part of
  the same metric family. I have added separate metrics for the
  capacity of queues as convenience. (They will never change and are
  only set once.)

- I left "metric_disk_latency_microseconds" unchanged, although that
  metric measures the latency of the storage device, even if it is not
  a spinning disk. "SSD" is read by many as "solid state disk", so
  it's not too far off. (It should be "solid state drive", of course,
  but "metric_drive_latency_microseconds" is probably confusing.)

- Brian suggested to not mix "failure" and "success" outcome in the
  same metric family (distinguished by labels). For now, I left it as
  it is. We are touching some bigger issue here, especially as other
  parts in the Prometheus ecosystem are following the same
  principle. We still need to come to terms here and then change
  things consistently everywhere.

Change-Id: If799458b450d18f78500f05990301c12525197d3