Add unique userid count for Geo proxied requests to service ping

What does this MR do and why?

This adds a custom header for Geo proxied events through Workhorse, and uses a HLLRedisCounter on the #show action for major controllers to store the unique users count going through the proxy.

Related to #346906 (closed)

Screenshots or screen recordings

How to set up and validate locally

Probably easiest using a GET Geo setup, then patching the Puma/Sidekiq nodes with this MR as per docs

1. Enable secondary proxying.
2. Go to the issue, project, MR pages with a specific user
3. Check that no events are recorded in Redis: Gitlab::Redis::HLL.count(keys: [Gitlab::UsageDataCounters::HLLRedisCounter::send(:redis_key, Gitlab::UsageDataCounters::HLLRedisCounter::send(:event_for, 'g_geo_proxied_requests'), Time.zone.now, '')])
4. Enable the feature flag using Feature.enable(:track_geo_proxy_events)
5. Navigate again to an issue / project / user, then notice the result of the HLL count above changes to 1.

MR acceptance checklist

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