Add Workhorse metrics for secondary proxy requests

What does this MR do and why?

Adds a gitlab_workhorse_http_geo_proxied_requests_total metric for proxied Geo requests, which gets aggregated into GeoNodeStatus as proxy_remote_requests_event_count_weekly and proxy_local_requests_event_count_weekly.

Related to #346906 (closed)

Screenshots or screen recordings

On a secondary, after a few manual requests:

How to set up and validate locally

On a GET setup (or local GDK+Geo+Unified URL+Prometheus - though I found that a bit more tricky to set up 😬):

1. run make in the workhorse/ directory on this branch
2. copy the gitlab-workhorse binary on the secondary (and primary to double check) in /opt/gitlab/embedded/bin/gitlab-workhorse
3. gitlab-ctl restart gitlab-workhorse
4. make a few requests through the unified URL from the secondary
5. go to url:9090/graph?g0.expr=gitlab_workhorse_http_geo_requests_total&g0.tab=1&g0.stacked=0&g0.range_input=1h and see the metrics

MR acceptance checklist

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