[O11y][PostgreSQL] Add label in Y-axis for `Database Overview` dashboard panels

[rajvi-patel-22]

Description

Add Y-axis unit for these two panels:

1. Local block cache stats
2. Shared block cache stats

• Add appropriate labels to Y-axis for Local block cache stats and Shared block cache stats
  • PR Link: [O11y][PostgreSQL] Lens migration for Database Overview visualizations to Kibana version 8.4.0 #5748

[rajvi-patel-22]

PR is approved and merged, hence closing this issue: #5748