auth: Declare ring-size metrics as gauges

auth: Declare ring-size metrics as gauges

ring-buffer size metrics are affected in three ways:

* incremented and saturated as items are added
* set to zero, when the ring-buffer is reset
* decremented when the ring-buffer is resized to a smaller capacity
  that cannot hold the number of items currently stored

The latter qualifies ring-buffer size metrics as gauges.