fix: Use a real true value for headers

[ajgajg1134]

What does this PR do?

Send "t" for the truth-y value for these http headers. No functional behavior change here. I picked "t" since it's small so there's a super micro optimization for ya (saved 4 bytes per payload 😎 )

Motivation

We use strconv.ParseBool in the trace-agent to read these headers, while it's true that we default to "true" when there's any non-empty value here that can't be parsed we do also log a debug message about it when we fail to parse a non-empty value. This is a super minor nitpick but it can make debug trace-agent logs very noisy when talking to the go tracer.

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 ./scripts/lint.sh locally.
• 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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-21 18:21:58

Comparing candidate commit efb7071 in PR branch andrew.glaude/smallTrueFix with baseline commit cd74ff7 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.