Update the Grafana dashboard that is included with Synapse in the `contrib` directory. (#13697)

* Add missing graph to contrib

* Update with minor but plausible changes, including positioning changes

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>