ddtrace/tracer: add IsTraceRoot to clients-side-stats

[knusbaum]

What does this PR do?

This PR adds the IsTraceRoot field to client stats payloads.

Motivation

In order to support adaptive sampling, we want to make this small change to the existing client stats implementation to bring it up to date with the version in the Trace Agent.

See:
DataDog/datadog-agent#23302
DataDog/datadog-agent#25374

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.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-21 16:54:45

Comparing candidate commit 2055b50 in PR branch knusbaum/is-trace-root-CSS with baseline commit e4985df in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 57 metrics, 1 unstable metrics.

scenario:BenchmarkInjectW3C-24

• 🟩 execution_time [-147.184ns; -110.216ns] or [-3.556%; -2.663%]

[felixge]

snake_case? 🙈

Not the end of the world, but I don't think we should let this creep into the code base further, cc @darccio as reviewer.

I submitted #2831 to clean up.

[darccio]

@felixge You are right, I didn't notice it. The trilean concept blinded me. We'll merge to improve this.