Add no-op asgi-lifespan handling

[sheenobu]

Super simple hook that checks for lifespan and immediately responds type.complete, essentially making startup/shutdown lifespan events no-ops for the prometheus ASGI app.

Fixes #855

from hypercorn.middleware import DispatcherMiddleware
from prometheus_client import make_asgi_app
import asyncio

async def main():

    from hypercorn.asyncio import serve
    from hypercorn.config import Config

    app = DispatcherMiddleware({
        "/metrics": make_asgi_app(),
    })


    config = Config()
    config.bind = ["localhost:8080"]

    await serve(app, config)

Before:

    assert scope.get("type") == "http"
AssertionError

After:

[2022-11-15 16:05:20 -0800] [59048] [INFO] Running on http://127.0.0.1:8080 (CTRL + C to quit)
INFO:hypercorn.error:Running on http://127.0.0.1:8080 (CTRL + C to quit)

Signed-off-by: Sheena Artrip [email protected]

[sheenobu]

If an exception is raised when calling the application callable with a lifespan.startup message or a scope with type lifespan, the server must continue but not send any lifespan events.

this might be a better solution!

[csmarchbanks]

If an exception is raised when calling the application callable with a lifespan.startup message or a scope with type lifespan, the server must continue but not send any lifespan events.

That seems nice, could we just change the assert to instead throw an exception? That would be nice for if other types show up in the future.