Fix(tracing): autodetect default Agent socket

[Seanbon0611]

Description

Add automatic detection of the default Datadog Agent Unix domain socket at /var/run/datadog/apm.socket.

When no Agent URL, host, or port is configured, the tracer now uses: unix:///var/run/datadog/apm.socket
if the socket path exists. Otherwise, it continues to use http://localhost:8126.
Configuration precedence remains:

1. Environment variables
2. Programmatic configuration
3. Default Agent socket
4. http://localhost:8126

Motivation

APMS-20562

Single-step instrumentation mounts the Agent socket at /var/run/datadog/apm.socket without necessarily setting DD_TRACE_AGENT_URL.
The C++ tracer previously ignored this socket and attempted to connect to localhost:8126, unlike other supported tracers. This prevented tracing from working in affected SSI environments.

Additional Notes

• Windows behavior is unchanged.
• Added deterministic tests for socket detection, fallback behavior, empty environment variables, and configuration precedence.
• Full test suite passes: 134 test cases.

Jira ticket: APMS-20562

[xlamorlette-datadog]

Neat!

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-09-25 09:39:44

Comparing candidate commit 1011db8 in PR branch Seanbon0611/autodetect_socket_path_when_unset with baseline commit d7a2e2a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 7 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 ----------------------------------'