[frontend] Add metric for in-flight requests on the frontend

Description

This MR adds a metric that exposes the number of in-flight requests in the CTA frontend. This metric only gives the view of a single point in time (when the metric is exported from CTA). Therefore, its granularity is dictated by the metrics export interval.

This MR also improves the way frontend requests are timed, making them less error proning (by using RAII).

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See our contributing docs

References

Closes #1322 (closed)