Make tracker statistics available via prometheus

[ldpr]

Currently the tracker stats are output in json, it'd be pretty neat if we could have them available via prometheus so we could hook them into things like grafana etc.

https://docs.rs/prometheus_exporter/latest/prometheus_exporter/

Maybe we could have a config option for json or prometheus?

Obviously this isn't important or anything, would be a "nice to have". Thanks for everything you do guys and girls! :)

[josecelano]

I think persisting statistics will also be a nice feature. Now statistics are reset when you restart the service.

[josecelano]

I found this docker compose configuration:

https://github.com/bitmagnet-io/bitmagnet/blob/32dc0bf6ec8db0a4aea0be1c59d7c8a17b13f268/docker-compose.yml

that might be useful to setup prometheus and grafana.

[josecelano]

I like what https://www.gbitt.info/ does:

[josecelano]

Hi, @ldpr; this has been implemented this week. See #1126. I have deployed it to the live demo and created this sample dashboard

I'm planning to write a blog post about it: torrust/torrust-website#131

And also to add some more metrics: #1128

cc @da2ce7