Bump version to 2.1.0, and fix ksr formatting

[xlamorlette-datadog]

In order to prepare a delivery, bump version to 2.1.0, because there are functional changes since the latest release, https://github.com/DataDog/dd-trace-cpp/releases/tag/v2.0.0 (from October 14th, 2025).

By the way, I had to fix the formatting of the Knuth sampling rate (_dd.p.ksr) to fixed, with maximum 6 decimal digits and no trailing decimal zeros.
This was recently (on March 12th) modified by #288.
But we just got new failures from system tests. See this failed CI job:

Expected _dd.p.ksr='0.000001', got: 1e-06
Expected _dd.p.ksr='0', got: 1e-07
Expected _dd.p.ksr='0.000001', got: 5.1e-07

[dubloom]

LGTM

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-05-05 14:27:13

Comparing candidate commit 6cf62cd in PR branch xlamorlette/version-2-1-0 with baseline commit 422dc91 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 0 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 ----------------------------------'

scenario:BM_TraceTinyCCSource

• 🟥 execution_time [+3.063ms; +3.216ms] or [+4.014%; +4.214%]

[datadog-prod-us1-5]

🎯 Code Coverage (details)
• Patch Coverage: 84.21%
• Overall Coverage: 90.86%

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 6cf62cd | Docs | Datadog PR Page | Give us feedback!}

[zacharycmontoya]

LGTM, thanks!

[bm1549]

Approving knuth sampling changes