Proposal: add (optional) ring support to overrides-exporter to only export metrics from 1 replica

[pracucci]

In the read-write deployment mode, the overrides-exporter run as part of the mimir-backend component. However, we don't need such metrics to be exported by all replicas but just from 1. Exporting these metrics from all replicas may end up to be high cardinality in a cluster with many tenants.

I propose to add an (optional) support for the ring in overrides-exporter, so that when configured only 1 instance will expose the metrics. This is the same technique used in #2957 to restrict query-scheduler component to a subset of replicas.