Add Grafana dashboard example

[levkk]

Description

Provide an example Grafana dashboard to help with self-hosted deployments.

[andrii29]

You can find an example dashboard here:
https://grafana.com/grafana/dashboards/24583-pgdog/

All panels use this prefix configuration:

[general]
openmetrics_namespace = "pgdog_"

I believe that the avg_received metric should be a gauge, similar to the other avg_ metrics.

avg_received Average number of bytes received. counter