Add the mimir service to the metrics catalog

metrics catalog, defining SLIs for the service to have MWMBR alerts and making sure the right saturation points apply. We need to make sure we have SLIs covering

• Remote write: Are we correctly receiving metrics from Prometheus
• Ruler: Are rule evaluations succeeding fast enough
• Querier: Can people query the data

The main service dashboard should be crosslinked with the mixin dashboards from #2635 (closed) by adding the right tags to all dashboards.