HTTPStatus.NOT_FOUND results in status "Hxx" when grouping (with fix)

[Leem0sh]

Hello,

just noticed a bug where any "non-numeric" status code for example

from http import HTTPStatus
from starlette.responses import JSONResponse


@router.post(...)
def f():
    ...
    return JSONResponse(status_code=HTTPStatus.NOT_FOUND, detail="Item not found")

results in

http_requests_total{handler="/myhandler",method="POST",status="Hxx"} 3.0

Easy fix in my code is to do int(HTTPStatus.NOT_FOUND).

Or you can probably fix it in https://github.com/trallnag/prometheus-fastapi-instrumentator/blob/master/src/prometheus_fastapi_instrumentator/middleware.py#L117

status = str(int(status))[0] + "xx"

?

[nikstuckenbrock]

I've created the PR #192 to fix this issue.