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 prometheus#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]>