Feature request: count exceptions

[v1lev]

Hello! Thank you for your work!

I'm currenlty migrating from starlette-prometheus due to the bug with BaseHTTPMiddleware. It has one very useful metric that counters all the exceptions per route: https://github.com/perdy/starlette-prometheus/blob/master/starlette_prometheus/middleware.py#L56

Please consider adding something like that in the future.

[dolfinus]

Isn't that the same as existing metric? starlette_requests_total{method="...",path="...",status_code="500"} 1.0