Add instrumentation to give insight into API request volume

There are probably other things we could instrument as well, but this would be a very good starting point.

We can export Prometheus metrics about API request volumes and later build support into https://github.com/brigadecore/brigade-metrics to scrape and display those metrics.

The issue names no files, tests, or entry points. Start by locating the API request handling path and any existing Prometheus instrumentation in Brigade. Define the request-volume metrics there, and consider the work done when those metrics can be exported for later scraping and display by brigade-metrics.

Written by the indexing model from the issue text.