feat(instrumentation/asgi): add target to metrics

[sk-]

Description

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.

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.

Partially fixes #1116

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

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

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)

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: sk- / name: Sebastian Kreft (086213c)

[sk-]

Not sure what to do regarding the failed lint checks

************* Module opentelemetry.instrumentation.asgi
instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py:449:4: R0912: Too many branches (13/12) (too-many-branches)
************* Module tests.test_asgi_middleware
instrumentation/opentelemetry-instrumentation-asgi/tests/test_asgi_middleware.py:1:0: C0302: Too many lines in module (1036/1000) (too-many-lines)

• How can I test the feature if there are no lines left to be added?
• One branch could be removed from the method __call__ by changing

for key, value in attributes.items():
  current_span.set_attribute(key, value)

to

current_span.set_attributes(attributes)

However, that's not even related to this PR.

I could also have a function that modifies the list of attributes, and hence moving the if target check to that function.

However, I think it'd be best to change collect_request_attributes to return a set of attributes suitable for metrics and another for traces. However, that would require a refactoring several places, as that function is public and actively used today.

[sk-]

@srikanthccv any advice regarding what to do to fix the lint violations?

See #1323 (comment)

It's complaining about too many lines in the test file and too many branches in the main function.

[srikanthccv]

Hm, I am not sure. I don't mind if you add pylint-disable.

[sk-]

Hm, I am not sure. I don't mind if you add pylint-disable.

Done, let me know if everything looks good now.