Improved the "Observability" section to better describe the Prometheus
metrics exposed by the Barman Cloud Plugin via the instance manager.
Added a mapping table in the "Verify your metrics" step to show the
transition from in-core metrics (`cnpg_collector_*`) to the new
plugin-based metrics (`barman_cloud_cloudnative_pg_io_*`).
Closes#473Closescloudnative-pg/cloudnative-pg#8902
Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
