Huge update of the Design documentation

[xlamorlette-datadog]

Description

This is a huge update of the “Datadog C++ Tracer Design” documentation. Notably:

• Add description of major missing features:
  • Metrics
  • Remote Configuration
  • Trace Context Propagation
  • Baggage Propagation
  • Trace/Span sampling
• Solve some inconsistencies with current implementation (which may come from code changes made since the documentation was written).
• Clean digressions about alternatives that were considered, to concentrate on current state.

Motivation

This documentation remained nearly untouched since its creation in August 2023. It was a bit cluttered with historical digressions, and it lacked the description of some salient features introduced in the meantime.

Additional Notes

The 'Components Relationships' diagram is lightly fixed in this PR, but it will get a major update in a following PR, to rather become a summarized class diagram.

Jira ticket: IDMPL-549 C++ Tracer: update and improve architecture documentation

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-07-09 09:43:40

Comparing candidate commit ee83302 in PR branch xlamorlette/update-design-documentation with baseline commit 6c29908 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 8 metrics, 0 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'