[metrics-team] Tor Weather: data sources

[Damian Johnson]

Thanks nusenu. I'm still unsure Onionoo makes sense but happy to leave
it to whoever makes the service.

> - how long is the relay down? (last_seen timestamp)

Unless I'm missing something if Onionoo derives this from 'is relay x
in the hourly consensus' then this is no different than 'download the
hourly consensus and if the relay's gone then note it'. You could get
a more precise measure by active probing (as already discussed) but
otherwise all this field provides you is information Weather already
knows.

> - is the relay running a recommended version? (recommended_version)

This is in the consensus.

> - give me all relays in this relays effective family (effective_family)

Gotcha. I'm unsure why Weather would require this but you're right
that if it is this could be a reason to use Onionoo.