ddtrace/tracer: add usr.id even with propagation

[Julio-Guerra]

What does this PR do?

Add the usr.id span tag even when the user id propagation is enabled.

Motivation

At first, we thought the backend would enrich this span tag based on the propagated _dd.p.usr.id one, but having usr.id present is part of the feature:

1. _dd.p.usr.id is set alone when it was found in the HTTP header for propagated tags (note how anyone can set it)
2. usr.id must always be set when using the SetUser() function, so that we can know that the user id has been programmatically set.

Describe how to test/QA your changes

• Go test from this PR
• System tests: https://github.com/DataDog/system-tests/blob/main/tests/test_identify.py (opening a PR to update them)

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[Julio-Guerra]

System-tests fix: DataDog/system-tests#817