fix(lib-injection): remove ddtrace stuff from path before adding to it [APMS-15139]

[emmettbutler]

This change addresses an issue in which Poetry running an app under a version of Python different from the one used to install Poetry causes library injection to fail. The approach is to ensure that the pythonpath doesn't contain anything related to ddtrace before adding new ddtrace-related entries to it.

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/clean-path-inject-42eb97e670d9ee67.yaml              @DataDog/apm-python
lib-injection/sources/sitecustomize.py                                  @DataDog/apm-core-python

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and main.

Summary

The average import time in this PR is: 238 ± 4 ms.

The average import time in main is: 239 ± 4 ms.

The import time difference between this PR and main is: -1.1 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.906 ms (0.80%)

ddtrace.bootstrap.sitecustomize 1.279 ms (0.54%)

ddtrace.bootstrap.preload 1.279 ms (0.54%)

ddtrace.internal.products 1.279 ms (0.54%)

ddtrace.internal.remoteconfig.client 0.624 ms (0.26%)

ddtrace 0.628 ms (0.26%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-21 17:08:16

Comparing candidate commit 4c6c160 in PR branch emmett.butler/clean-path-inject with baseline commit 00c254b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 498 metrics, 2 unstable metrics.