Add optionnal simple telemetry so we now how many docs instances are out there

[virgile-dev]

We recently added a table of know instances on the README.
People are encouraged to add theirs via a PR.
Ideally we'd love to be able to know how many instances are out there without having to ask people to do a PR.
The way I see it Docs instances could send a ping and a couple info (Docs version, Number of users, of Docs).
This ping would be optional and could be disabled by setting and env variable to false

[rvveber]

Since we need to get that data from the backend anyways, it would make sense to revive #562 where that data is already collected at the backend and already exposed at a special url.

Then for transmitting this data to you, all you would need to do, is to communicate this special url to you central server.
From there you could query every url and get all the data feeded and visually displayed into for example a grafana dashboard.

[virgile-dev]

I like it ! @lunika your take ?

[rvveber]

The access to this special url is controlled by a CIDR (List of IP-Adresses or IP-Ranges).
We could have an ENV variable that if true, would add your central server IP to the CIDR internally.
As far as i can see from #562 all metrics exposed are numbers only, no sensitive data is monitored.
This means we could probably allow this for most openDesk instances.