Add a prometheus exporter to moat distributor

Let's collect prometheus metrics on the Circumvention Settings moat distributor. We might want to collect metrics for:

• Requests to settings with country and valid shim token as labels
• Requests to other API endpoints with the endpoint as label (settings, defaults, map, builtin)

Some inspiration can be taken from how is the prometheus exporter in gettor: https://gitlab.torproject.org/tpo/anti-censorship/rdsys/-/blob/main/pkg/usecases/distributors/gettor/gettor.go