fix(structlog): ensures processor injection is not duplicated [backport 2.7]

[github-actions]

Backport 6f1ecfd from #8713 to 2.7.

Fixes #8699

Note that when the user has a configuration with no processors, the instrumentation does not inject any processors since there must be a rendered processor for a valid configuration

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.
• If change touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• 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

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-8713-to-2.7
Commit report: b0d06ce
Test service: dd-trace-py

✅ 0 Failed, 192 Passed, 0 Skipped, 2m 47.56s Total duration (3.72s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-03-25 16:40:32

Comparing candidate commit b0d06ce in PR branch backport-8713-to-2.7 with baseline commit 94ded6f in branch 2.7.

Found 0 performance improvements and 3 performance regressions! Performance is the same for 198 metrics, 9 unstable metrics.

scenario:httppropagationextract-empty_headers

• 🟥 max_rss_usage [+456.410KB; +735.526KB] or [+2.146%; +3.459%]

scenario:httppropagationextract-wsgi_empty_headers

• 🟥 max_rss_usage [+437.676KB; +780.884KB] or [+2.082%; +3.714%]

scenario:httppropagationextract-wsgi_invalid_trace_id_header

• 🟥 max_rss_usage [+897.420KB; +974.042KB] or [+4.291%; +4.658%]