prometheus: add matrix metrics and dashboard

The dashboards is from [1] and fixed with:
sed 's/${DS_PROMETHEUS}/$datasource/g' -i synapse.json

[1] https://github.com/matrix-org/synapse/tree/c167e09fe58d3a256fb1c763b391ad6633d2507d/contrib/grafana

Closes: #290

Signed-off-by: Leonidas Spyropoulos <artafinde@gmail.com>
2 jobs for prometheus_matrix_metrics in 33 seconds (queued for 7 seconds)