policy: add selector cache metrics

policy: add selector cache metrics

This adds a new set of metrics for the selector cache. Both in terms of cardinality,
but also around the performance of various operations.

Contention in the selector cache can easily make regenerations slower,
and potentially grind the whole agent to a halt in special cases. This
allows us to use these metrics to better observe the performance of
these operations.

Signed-off-by: Odin Ugedal <odin@ugedal.com>
Signed-off-by: Odin Ugedal <ougedal@palantir.com>