fix: _dd.p.ksr formatting to use 6 significant digits without trailing zeros

[bm1549]

Description

Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros, using std::to_chars with std::chars_format::general. Also skip setting _dd.p.ksr when the sampling mechanism is DEFAULT (no agent configuration received yet).

Changes:

• Replace snprintf with std::to_chars for _dd.p.ksr formatting
• Use std::chars_format::general with 6 significant digits (matches Go tracer behavior)
• Skip setting _dd.p.ksr when SamplingMechanism::DEFAULT is active
• Add error handling for std::to_chars conversion failures
• Update tests to expect the new formatting

Motivation

The previous snprintf approach with %g format produced inconsistent output compared to other tracers. std::to_chars with std::chars_format::general gives us precise control over significant digits while automatically removing trailing zeros.

Additionally, setting _dd.p.ksr with the DEFAULT mechanism is meaningless since no sampling configuration has been received from the agent yet.

Additional Notes

Jira ticket: [PROJ-IDENT]

🤖 Generated with Claude Code

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-12 15:33:47

Comparing candidate commit d9275b8 in PR branch brian.marks/fix-ksr-formatting with baseline commit f8c3913 in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 0 metrics, 0 unstable metrics.

scenario:BM_TraceTinyCCSource

• 🟩 execution_time [-2.357ms; -2.111ms] or [-3.017%; -2.701%]

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 58.33%
• Overall Coverage: 87.70% (-0.07%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: d9275b8 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.70%. Comparing base (f8c3913) to head (d9275b8).

Files with missing lines	Patch %	Lines
src/datadog/trace_segment.cpp	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
- Coverage   87.76%   87.70%   -0.07%     
==========================================
  Files          84       84              
  Lines        5658     5668      +10     
==========================================
+ Hits         4966     4971       +5     
- Misses        692      697       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dmehala]

Well done.

EDIT: I'm trying to enforce semantic commits, please prefix your commit message with fix: during the merge. Thanks.