Add a connection listener that emits telemetry events

[v0idpwn]

Alternative to #310. Would have to write documentation, etc.

[v0idpwn]

I added two documentation sections. I'm afraid it's a bit hidden in the documentation, but the DBConnection module doc is already quite big :)

I also considered starting one of these on DBConnection supervision tree, with a name, to make it easier to configure (no need to start a process), but I guess it's not worth it, since most people won't use it.

[v0idpwn]

Almost merged it as DbConnection instead of DBConnection 😄

[v0idpwn]

@josevalim I would love if this could be released soon, it would help us at @coingaming to improve an alarm.

[josevalim]

Done!