feat: add counter apdexes for reference arch monitoring

Closes #109 (closed).

This updates reference architecture monitoring to rely on custom latencies instead of fixed latencies.

This may help with spurious SLO alerts in single-tenant environments, such as the one reported in https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/1725.

Comparing Results

In incident https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/1725, we saw apdex drop to 98% for well over 2 hours as queries hit the otherwise quiet instance.

With this change, almost all the requests are within the satisfactory latency as specified by the stage group teams that own these endpoints: