feat(httptrace): add query string parameter allowlist via DD_TRACE_HTTP_URL_QUERY_STRING_ALLOWLIST

[genesor]

What does this PR do?

Adds a new environment variable DD_TRACE_HTTP_URL_QUERY_STRING_ALLOWLIST that lets users specify a comma-separated list of query parameter keys to retain in the http.url span tag. When set, only the listed keys are kept and the expensive default obfuscation regex is bypassed entirely in favor of a simple split-and-filter pass over the raw query string.

Motivation

A customer on dd-trace-go v2.5 reported ~70% CPU overhead caused by the default query string obfuscation regex in URLFromRequest running on every HTTP span. Their query strings are large. Disabling query strings entirely (DD_TRACE_HTTP_URL_QUERY_STRING_DISABLED=true) was not viable since they need specific parameters for debugging. The customer requested the ability to pick specific query parameter keys to keep (e.g. [p1, p3]) with high performance.

Related: APMS-18777

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

[datadog-prod-us1-4]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 72.00%
• Overall Coverage: 59.39% (+3.51%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: b89c3f1 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

[codecov]

Codecov Report

❌ Patch coverage is 73.07692% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.67%. Comparing base (206b642) to head (b89c3f1).
⚠️ Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
instrumentation/httptrace/config.go	0.00%	6 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
instrumentation/httptrace/httptrace.go	90.90% <100.00%> (-2.08%)	⬇️
instrumentation/httptrace/config.go	82.82% <0.00%> (-9.68%)	⬇️

... and 432 files with indirect coverage changes

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[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-18 23:38:00

Comparing candidate commit b89c3f1 in PR branch ben.db/feat-query-string-allowlist with baseline commit 206b642 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 156 metrics, 8 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 ----------------------------------'