feat(common): allocation-free tag validation and parsing

[morrisonlevi]

What does this PR do?

Adds typed tag validation errors and a borrowed iterator for tag lists.

Motivation

In the OTel process and context work in PHP, we are now going to be making tags on every sample. I'm working to avoid allocations, or delay them until later if feasible. Right now there's no room because the only validation APIs are to actually make a tag. These new APIs let us validate key:value, value, and DD_TAGS style tags all without allocating.

Additional Notes

These new APIs are consumed internally so they aren't dead code either, except for validating a (key, value) pair. The reason is that the way it is technically implemented in the constructor is to format!("{key}:{value}") and then use the serialized key:value validator instead. This is undesirable for me so I've added the new API and tried to make property tests to ensure it stays in-sync with the equivalent behavior.

How to test the change?

Test regularly, nothing should be significantly different.

[datadog-prod-us1-4]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 75.92% (+0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: e1489d3 | Docs | Datadog PR Page | Give us feedback!}

[dd-octo-sts]

Artifact Size Benchmark Report

aarch64-alpine-linux-musl

Artifact	Baseline	Commit	Change
/aarch64-alpine-linux-musl/lib/libdatadog_profiling.a	89.19 MB	89.19 MB	-0% (-6.21 KB) 👌
/aarch64-alpine-linux-musl/lib/libdatadog_profiling.so	8.20 MB	8.20 MB	0% (0 B) 👌

aarch64-unknown-linux-gnu

Artifact	Baseline	Commit	Change
/aarch64-unknown-linux-gnu/lib/libdatadog_profiling.so	11.06 MB	11.06 MB	+0% (+128 B) 👌
/aarch64-unknown-linux-gnu/lib/libdatadog_profiling.a	100.37 MB	100.36 MB	-0% (-6.10 KB) 👌

libdatadog-x64-windows

Artifact	Baseline	Commit	Change
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.dll	26.63 MB	26.63 MB	-0% (-1.00 KB) 👌
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.lib	94.56 KB	94.56 KB	0% (0 B) 👌
/libdatadog-x64-windows/debug/dynamic/datadog_profiling_ffi.pdb	191.43 MB	191.42 MB	-0% (-8.00 KB) 👌
/libdatadog-x64-windows/debug/static/datadog_profiling_ffi.lib	1001.70 MB	1001.43 MB	--.02% (-272.57 KB) 💪
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.dll	8.71 MB	8.71 MB	--.02% (-2.00 KB) 💪
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.lib	94.56 KB	94.56 KB	0% (0 B) 👌
/libdatadog-x64-windows/release/dynamic/datadog_profiling_ffi.pdb	25.62 MB	25.62 MB	0% (0 B) 👌
/libdatadog-x64-windows/release/static/datadog_profiling_ffi.lib	51.01 MB	51.01 MB	-0% (-2.73 KB) 👌

libdatadog-x86-windows

Artifact	Baseline	Commit	Change
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.dll	23.22 MB	23.22 MB	-0% (-1.00 KB) 👌
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.lib	96.04 KB	96.04 KB	0% (0 B) 👌
/libdatadog-x86-windows/debug/dynamic/datadog_profiling_ffi.pdb	196.35 MB	196.33 MB	--.01% (-24.00 KB) 💪
/libdatadog-x86-windows/debug/static/datadog_profiling_ffi.lib	990.83 MB	990.81 MB	-0% (-23.72 KB) 👌
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.dll	6.73 MB	6.73 MB	--.01% (-1.00 KB) 💪
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.lib	96.04 KB	96.04 KB	0% (0 B) 👌
/libdatadog-x86-windows/release/dynamic/datadog_profiling_ffi.pdb	27.55 MB	27.55 MB	0% (0 B) 👌
/libdatadog-x86-windows/release/static/datadog_profiling_ffi.lib	48.56 MB	48.55 MB	-0% (-2.20 KB) 👌

x86_64-alpine-linux-musl

Artifact	Baseline	Commit	Change
/x86_64-alpine-linux-musl/lib/libdatadog_profiling.a	79.47 MB	79.46 MB	--.01% (-8.64 KB) 💪
/x86_64-alpine-linux-musl/lib/libdatadog_profiling.so	9.13 MB	9.12 MB	--.04% (-4.00 KB) 💪

x86_64-unknown-linux-gnu

Artifact	Baseline	Commit	Change
/x86_64-unknown-linux-gnu/lib/libdatadog_profiling.a	95.12 MB	95.12 MB	-0% (-8.42 KB) 👌
/x86_64-unknown-linux-gnu/lib/libdatadog_profiling.so	11.14 MB	11.14 MB	--.03% (-3.88 KB) 💪

[pr-commenter]

Benchmarks

Comparison

Benchmark execution time: 2026-08-07 05:43:06

Comparing candidate commit e1489d3 in PR branch PROF-15487-allocation-free-tag-validation with baseline commit ea75b04 in branch main.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 138 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:vec_map/as_deduped_map/already_deduped/16

• 🟥 execution_time [+2.722ns; +3.818ns] or [+8.940%; +12.538%]

scenario:vec_map/get_miss/64

• 🟥 execution_time [+28.678ns; +28.785ns] or [+102.664%; +103.045%]

Benchmark execution time: 2026-08-07 05:54:01

Comparing candidate commit e1489d3 in PR branch PROF-15487-allocation-free-tag-validation with baseline commit ea75b04 in branch main.

Found 7 performance improvements and 2 performance regressions! Performance is the same for 171 metrics, 10 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:alloc_free/system/4096

• 🟩 execution_time [-12.315ns; -12.125ns] or [-11.627%; -11.448%]

scenario:datadog_sample_span/name_pattern_rule_matching/wall_time

• 🟥 execution_time [+33.482ns; +33.666ns] or [+13.042%; +13.114%]

scenario:profiler_attached/fast_path_system/16

• 🟩 execution_time [-17.191ns; -17.162ns] or [-38.611%; -38.546%]

scenario:profiler_attached/fast_path_system/256

• 🟩 execution_time [-17.283ns; -17.249ns] or [-38.811%; -38.736%]

scenario:profiler_attached/fast_path_system/64

• 🟩 execution_time [-17.317ns; -17.282ns] or [-38.890%; -38.810%]

scenario:profiler_attached/fast_path_system/65536

• 🟩 execution_time [-10.084ns; -9.922ns] or [-9.029%; -8.883%]

scenario:profiler_attached/slow_path_system/4096

• 🟥 execution_time [+25.134ns; +25.236ns] or [+19.659%; +19.739%]

scenario:trace_buffer/4_senders/no_delay

• 🟩 execution_time [-180.039µs; -155.845µs] or [-6.806%; -5.892%]
• 🟩 throughput [+85566.255op/s; +99006.913op/s] or [+6.284%; +7.271%]

Candidate

Omitted due to size.

Baseline

Omitted due to size.

[morrisonlevi]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-08-10 14:00:52 UTC ℹ️ Start processing command /merge

---

2026-08-10 14:00:56 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in main is approximately 53m (p90).

---

2026-08-10 15:18:03 UTC ℹ️ MergeQueue: This merge request was merged