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

## Description
Add Y-axis unit for these two panels:

1. Local block cache stats
2. Shared block cache stats

- [x] Add appropriate labels to Y-axis for `Local block cache stats` and `Shared block cache stats`
   - PR Link: https://github.com/elastic/integrations/pull/5748

![image](https://user-images.githubusercontent.com/101976829/229465364-a6cf99d0-f248-45f0-9335-4a9f014680ca.png)