feat(grafana): add index io and usage panels

Add new single-index panels for size, reads/hits (bytes),
index scans, and tuples read/fetched.

Extend aggregated index analysis with top-N panels for
reads/hits (bytes) and tuples read/fetched.

Include datname label in pg_stat_all_indexes metric so
dashboards can filter by db_name.

Files changed:
- config/grafana/dashboards/Dashboard_10_Index health.json
- config/grafana/dashboards/Dashboard_11_Single_index_analysis.json
- config/pgwatch-prometheus/metrics.yml

Author: dementii-priadko