Add http.target to Django duration metric attributes

[alexmojaki]

Description

This is similar to #2621 which is for #1457, but this PR is for Django rather than Flask.

It adds the http.target attribute to the metric named http.server.duration.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

"New feature" feels right and I've chosen it to match #2621 and its changelog update. Arguably this is a bug fix because the attribute is required by the spec, and technically it could be considered a breaking change because this will increase the number of metric data points emitted, but the attribute has low cardinality.

How Has This Been Tested?

The existing test failed until I updated it to also expect the http.target attribute.

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: alexmojaki / name: Alex Hall (dc42bdd, 6cda892, 259531b, 26d9110, 1bc4146, 42f673c, 00bf613, 6a7c0a1)

[alexmojaki]

The way I've done this may also affect other instrumentations that use _duration_attrs in util/opentelemetry-util-http/src/opentelemetry/util/http/__init__.py if those instrumentations set http.route in the right place.

I've just found that the FastAPI tests fail because of this: the metrics now have both http.route and http.target with the same value. What should be done about this? I listed some options in #1457 (comment)

Changed http.route to http.target

[alexmojaki]

http.route is also appearing in the starlette metrics, which is causing those tests to fail, but that just means that this fix applies to both starlette and django.

[alexmojaki]

This should be mergeable now. Only thing I'm unsure about is the changelog entry. This doesn't need a documentation update, right?

[lzchen]

Thanks for the change @alexmojaki ! I know the semantic convention stuff was confusing haha.