Add a prometheus gauge metric that tracks the amount of in flight requests. Using a middleware, we would increment the gauge, and decrement it with a deferred function.

This metric will be useful to detect strange behavior (like websocket spam) or something, which will be super useful to alert on especially once we have #16904.

If it's easy, this should also label by route pattern e.g. route="/api/v2/workspaceagents/:id or something. Don't include IDs in the metrics to avoid ballooning labels.