nwaku-monitor/pg-exporter-queries: add new panel to show num msgs per shard

[Ivansete-status]

In this PR we are showing the number of messages per shard.

The original query (monitoring/configuration/pg-exporter-queries.yml) was designed by @fryorcraken

As a result, we add a panel with the following shape:

[NagyZoltanPeter]

What has really changed in nwaku-monitoring.json file, it's hard to see... Maybe some shift cannot be interpreted properly by diff.

[gabrielmer]

Greatt, thanks!

[gabrielmer]

What has really changed in nwaku-monitoring.json file, it's hard to see... Maybe some shift cannot be interpreted properly by diff.

Yes, it's impossible to properly review changes in Grafana dashboards, the diff is always a mess

[Ivansete-status]

@NagyZoltanPeter , @gabrielmer - Yes, I absolutely agree that is very difficult to track the changes because that file is modified from the Grafana UI and when moving different components, it gets changed in many places.

When changing the board is a good idea to indicate in the PR description which component in being added.

In this case, you need to trust me that I'm just adding the "# messages per shard" component :)