Fix handling root_path

[dolfinus]

1. Fix handling of applications started using uvicorn --root-path=/api. Fixes filter_unhandled_paths broken with root_path set via Uvicorn #75.

Previously PrometheusMiddleware just threw away root_path from request scope. Because of this, _get_router_path always returned None, and if PrometheusMiddleware was used with both group_paths=True and filter_unhandled_paths=True, all application endpoints were just excluded from monitoring.

This requires statlette>=0.33 because of Kludex/starlette#2352.

2. Previously __call__ method was implemented like this:

try:
  await self.app(scope, receive, send)
except Exception:
  ...
  raise
finally:
  if is_known_endpoint(path):
    return
  ...

But if the finally clause executes a return statement, exceptions are not re-raised. So bug from item 1 prevented unhandled exceptions from being raised properly. This caused some unexpected log messages, like this:

Simple application

from fastapi import FastAPI

app = FastAPI()

@app.get("/data")
def get_data():
    raise RuntimeError

from starlette_exporter import PrometheusMiddleware

app.add_middleware(
    PrometheusMiddleware,
    app_name="my_app",
    group_paths=True,
    filter_unhandled_paths=True,
)

uvicorn main:app --port 8080 --root-path /api

curl http://localhost:8080/data

INFO:     Started server process [64935]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8080 (Press CTRL+C to quit)
ERROR:    ASGI callable returned without starting response.
INFO:     127.0.0.1:58474 - "GET /data HTTP/1.1" 500 Internal Server Error

I've just removed finally block:

exception = None
try:
  await self.app(scope, receive, send)
except Exception as e:
  ...
  exception = e

if is_known_endpoint(path):
  if exception:
    raise exception
  return

...

if exception:
  raise exception

Fixes #80.

3. Added tests for custom root_path with failing endpoints and not, with enabled filter_unhandled_paths and not, to cover all possible cases. Also rearranged few tests a bit.

[stephenhillier]

@dolfinus thanks for the PR. This looks like a good solution. For the new base path test, it looks like route matching is failing - self._get_router_path(scope) on line 340 might be returning None. Any ideas? I'll try to take a look as well over the next few days.

[dolfinus]

This test is failing on starlette<0.33 because of Kludex/starlette#2352. Added condition here.

It is possible to update minimal version of starlette to fix this, but FastAPI still does not support this version: fastapi/fastapi#10798

[stephenhillier]

Thank you! Really appreciate the contribution and explanation.