chore(tracer): make ddtrace.Tracer a singleton

[mabdinur]

• Logs a deprecation warning if the Tracer is initialized more than once.
• Ensures only one instance of the tracer exists per process
• Ensures that multiple instances of the DummyTracer can be created (maintain the current behavior). This will help reset tracer configurations between tests.

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/deprecate-multiple-tracer-instances-078b920081ba4a36.yaml  @DataDog/apm-python
ddtrace/_trace/tracer.py                                                @DataDog/apm-sdk-api-python
ddtrace/contrib/grpc/__init__.py                                        @DataDog/apm-core-python @DataDog/apm-idm-python
ddtrace/contrib/vertica/__init__.py                                     @DataDog/apm-core-python @DataDog/apm-idm-python
ddtrace/opentracer/tracer.py                                            @DataDog/apm-sdk-api-python
tests/utils.py                                                          @DataDog/python-guild

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: munir/make-tracer-a-singleton
Commit report: 4f93e15
Test service: dd-trace-py

✅ 0 Failed, 1233 Passed, 235 Skipped, 19m 50.78s Total duration (16m 8.69s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-12-20 21:46:42

Comparing candidate commit 728a816 in PR branch munir/make-tracer-a-singleton with baseline commit 42f69dc in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 394 metrics, 2 unstable metrics.