This PR adds the target information for metrics reported by instrumentation/asgi.

Unfortunately, there's no ASGI standard to reliably get this information, and I was only able to get it for FastAPI.

I also tried to get the info with Sanic and Starlette (encode/starlette#685), but there's nothing in the scope allowing to recreate the route.

Besides the included unit tests, the logic was tested using the following app:

```python
import io
import fastapi

app = fastapi.FastAPI()

def dump_scope(scope):
    b = io.StringIO()
    print(scope, file=b)
    return b.getvalue()

@app.get("/test/{id}")
def test(id: str, req: fastapi.Request):
    print(req.scope)
    return {"target": _collect_target_attribute(req.scope), "scope": dump_scope(req.scope)}

sub_app = fastapi.FastAPI()

@sub_app.get("/test/{id}")
def sub_test(id: str, req: fastapi.Request):
    print(req.scope)
    return {"target": _collect_target_attribute(req.scope), "scope": dump_scope(req.scope)}

app.mount("/sub", sub_app)
```

Partially fixes open-telemetry#1116

Note to reviewers: I tried to touch as less as possible, so that we don;t require a refactor before this change. However, we could consider changing `collect_request_attributes` so that it returns both a trace attributes and a metrics attributes.
Wihout that change we cannot add the `HTTP_TARGET` attribute to the list of metric atttributes, because it will be present but with high cardinality.

…asgi_middleware.py

Co-authored-by: Srikanth Chekuri <srikanth.chekuri92@gmail.com>

…emetry/instrumentation/asgi/__init__.py

Co-authored-by: Srikanth Chekuri <srikanth.chekuri92@gmail.com>