fix(tracing): only apply rate limits when trace sample rules are set [except for ASM Standalone] [backport 2.14]

[github-actions]

Backport afb6f2f from #10764 to 2.14.

Aligns the implementation of DD_TRACE_RATE_LIMIT with the java, dotnet and nodejs tracers: https://docs.datadoghq.com/tracing/trace_collection/library_config/java/#configuration-options.

Linked to: https://github.com/DataDog/dd-trace-py/pull/10830/files.

Changes

• When tracing is enabled only apply DD_TRACE_RATE_LIMIT if a trace sampling rules or a trace sample rate is set. Otherwise trace rate limiting should be handled by the Datadog Agent.
• [backwards compatibility] When ASM is enabled in standalone mode, rate limiting of 1 trace per second should be applied to ALL spans. All rate limited spans should have priority category of USER.
• When spans are rate limited set _dd.limit_psr to the effective rate limit. Previously this tag was only set is DD_TRACE_RATE_LIMIT != 100. This restriction is not found in the spec.

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

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-10764-to-2.14
Commit report: f976379
Test service: dd-trace-py

✅ 0 Failed, 1286 Passed, 0 Skipped, 35m 35.72s Total duration (43.93s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-04 00:06:37

Comparing candidate commit f976379 in PR branch backport-10764-to-2.14 with baseline commit 7fcc3a0 in branch 2.14.

Found 3 performance improvements and 0 performance regressions! Performance is the same for 353 metrics, 48 unstable metrics.

scenario:span-start-finish

• 🟩 execution_time [-5.857ms; -5.365ms] or [-10.620%; -9.727%]

scenario:span-start-finish-telemetry

• 🟩 execution_time [-6.123ms; -5.610ms] or [-10.816%; -9.909%]

scenario:span-start-finish-traceid128

• 🟩 execution_time [-6.143ms; -5.482ms] or [-10.459%; -9.333%]

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/only-rate-limit-on-sampling-rules-6f807defa7d0f5f1.yaml  @DataDog/apm-python
ddtrace/_trace/tracer.py                                                @DataDog/apm-sdk-api-python
ddtrace/internal/rate_limiter.py                                        @DataDog/apm-core-python
ddtrace/sampler.py                                                      @DataDog/apm-sdk-api-python
ddtrace/settings/config.py                                              @DataDog/python-guild @DataDog/apm-sdk-api-python
docs/configuration.rst                                                  @DataDog/python-guild
tests/integration/test_sampling.py                                      @DataDog/apm-core-python
tests/snapshots/test_extended_sampling_glob_multi_rule.json             @DataDog/apm-python
tests/snapshots/test_sampling_with_default_sample_rate_1.json           @DataDog/apm-python
tests/snapshots/test_sampling_with_default_sample_rate_1_and_rule_1.json  @DataDog/apm-python
tests/snapshots/test_sampling_with_rate_limit_3.json                    @DataDog/apm-python
tests/snapshots/test_sampling_with_sample_rate_1_and_rate_limit_3_and_rule_0.json  @DataDog/apm-python
tests/snapshots/tests.integration.test_trace_stats.test_single_span_sampling[sampling_rule1].json  @DataDog/apm-python
tests/tracer/test_sampler.py                                            @DataDog/apm-sdk-api-python