chore(tracer): deprecate multiple initializations of ddtrace.Tracer [3.0]

[mabdinur]

• Logs a deprecation warning if the Tracer is initialized more than once. The global tracer.
• Ensures that multiple instances of the DummyTracer and CIVisibility Tracer can be created (maintain the current behavior). This will help reset tracer configurations between tests.
• Update some tests that require multiple tracer instances. This will make the 3.0 upgrade easier to do.

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
tests/snapshots/tests.integration.test_propagation.test_trace_tags_multispan.json  @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/internal/ci_visibility/recorder.py                              @DataDog/ci-app-libraries
ddtrace/opentracer/tracer.py                                            @DataDog/apm-sdk-api-python
tests/integration/test_context_snapshots.py                             @DataDog/apm-core-python
tests/integration/test_debug.py                                         @DataDog/apm-core-python
tests/integration/test_encoding.py                                      @DataDog/apm-core-python
tests/integration/test_integration.py                                   @DataDog/apm-core-python
tests/integration/test_integration_civisibility.py                      @DataDog/ci-app-libraries
tests/integration/test_integration_snapshots.py                         @DataDog/apm-core-python
tests/integration/test_priority_sampling.py                             @DataDog/apm-sdk-api-python
tests/integration/test_propagation.py                                   @DataDog/apm-sdk-api-python
tests/integration/test_sampling.py                                      @DataDog/apm-core-python
tests/integration/test_settings.py                                      @DataDog/apm-core-python
tests/integration/test_trace_stats.py                                   @DataDog/apm-core-python
tests/integration/test_tracemethods.py                                  @DataDog/apm-core-python
tests/integration/utils.py                                              @DataDog/apm-core-python
tests/profiling/test_profiler.py                                        @DataDog/profiling-python
tests/snapshots/tests.integration.test_integration_snapshots.test_tracetagsprocessor_only_adds_new_tags.json  @DataDog/apm-python
tests/tracer/test_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: e6d9b45
Test service: dd-trace-py

✅ 0 Failed, 1048 Passed, 550 Skipped, 18m 22.29s Total duration (21m 38.36s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-15 22:07:42

Comparing candidate commit e6d9b45 in PR branch munir/make-tracer-a-singleton with baseline commit 13b1457 in branch main.

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

[P403n1x87]

Would we be able to manage the global tracer with the product plugin interface now? If so, can we make that change in this PR as well?

[ZStriker19]

I think this should have a deprecation release note. Other than that just one question about testing changes.

[mabdinur]

I think this should have a deprecation release note. Other than that just one question about testing changes.

It already does 😄