PrometheusMiddleware raises wrong exception

[MrBTTF]

Hi
If a route handler fails, PrometheusMiddleware raises this kind of exception

...
  File "/app/common/server.py", line 55, in middleware
    response = await call_next(request)
  File "/usr/local/lib/python3.9/site-packages/starlette/middleware/base.py", line 45, in call_next
    task.result()
  File "/usr/local/lib/python3.9/site-packages/starlette/middleware/base.py", line 38, in coro
    await self.app(scope, receive, send)
  File "/usr/local/lib/python3.9/site-packages/starlette_exporter/middleware.py", line 105, in __call__
    self.request_time.labels(*labels).observe(end - begin)
TypeError: unsupported operand type(s) for -: 'NoneType' and 'float'

It is the result of end being not initialized on this line
https://github.com/stephenhillier/starlette_exporter/blob/master/starlette_exporter/middleware.py#L110
if wrapped_send function is not called due to exception on this line
https://github.com/stephenhillier/starlette_exporter/blob/master/starlette_exporter/middleware.py#L86
Apparently wrapper_send is not called because startlette raises exception from a route handler function before calling sender
https://github.com/encode/starlette/blob/master/starlette/exceptions.py#L82

[Steve Hillier]

Hi @MrBTTF, f81ee3c was supposed to fix this. Did you get a chance to test v0.8.1 or is the problem still happening?
ps, thanks for the all the details. If you're still having an issue that'll help narrow it down.

[MrBTTF]

Now the proper exception is raised. Thanks!