mgr/prometheus:Improve the pool metadata

mgr/prometheus:Improve the pool metadata

Adds percent_used and used_bytes to the per pool
metrics group, and add additional labels to the ceph_pool_metadata
metric
- compression_mode - so consumers can tell whether compression
  is active
- description - provide a text string showing the protection
  scheme of the pool (e.g. replica2, or ec:4+2
- type - text string showing replicated or erasure enabling
  easy filtering across the pools

These additional fields allow compression savings to be more
easily shown. The inclusion of percent_used ensures that any
prometheus based view of pool usage will ALWAYS match the CLI.

Fixes: https://tracker.ceph.com/issues/49049
Signed-off-by: Paul Cuzner <pcuzner@redhat.com>