track http service error count

[daniel-salib]

Track error count at the http service level in the app state.

Register as aggregated_error_count in the prometheus metrics

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[jmswen]

This is a bit of a noob question – I'm not super familiar with FastAPI – but would it be cleaner to track 4xx/5xx responses via some custom middleware (e.g., app.add_middleware(ResponseCodeMiddleware))? That way, it's easier to reason about the code and make sure we don't miss a spot where the counter should be incremented.

[daniel-salib]

yea you're right, middleware is the cleanest way to handle this - however we've previously found when developing load_aware_call that the middleware approach negatively impacts performance. When running the benchmark with the decorator instead we don't see the same performance hit

[youngkent]

I view http-middleware decorator as a code structure extension instead of feature extension. No need to have a explicit control for this?
The feature we could control is the counter/logging, but I feel it's pretty standard and we should just enable it for all use cases? cc: @simon-mo

[simon-mo]

cc @markmc
Should we consider external package like this? https://github.com/stephenhillier/starlette_exporter

[daniel-salib]

cc @markmc Should we consider external package like this? https://github.com/stephenhillier/starlette_exporter

prometheus_fastapi_instrumentator seems more popular and is already included in the common vllm requirements

I went ahead and implemented it here:
#15657