feat(matrix): Add dashboard to further analyse the synapse mystery

After digging a bit deeper into the data, it's clear that there is a
correlation between large volume of DNS requests and the `GET` requests
to the `_matrix` route.

When these slow down, the DNS requests suddenly also disappear. It's
noteworthy, that `PUT` requests (means incoming Events), remain
unchanged.

The potential releation became apparent, when on 2022-04-30 during the
evening hours, there is a 21:30 to 01:55 CEST time window where this
correlation becoming apparant.

The `GET` request can be more drilled down using the Synapse Dashboard
to be `SyncRestServlet` requests. These requests are client requests, to
keep the client state up-to-date and active and explains the lack of
requests for a few hours, when Element Desktop was shut down.

This needs some further drill down into Synapse's internals to figure
out exactly what causes these mass request, but `presence` is a good
contender.

References:
https://github.com/matrix-org/synapse/blob/0922462fc7df951e88c8ec0fb35e53e3cd801b76/synapse/rest/client/sync.py#L52
https://github.com/matrix-org/synapse/tree/8d156ec0ba17d848581f18aa40ebfd76dda763d4/contrib/grafana