tracer: honor tracecontext propagation style precedence

[purple4reina]

What does this PR do?

This PR changes the precedence of the tracecontext propagation style to match what is requested by the user exactly. Previously, this propagation style was always set as the first style, even if the user specifies it later in the chain.

Motivation

This new behavior aligns with the behavior specified in our public docs and with other language tracers.

If multiple extraction styles are enabled, extraction attempts are made in the order that those styles are specified. The first successfully extracted value is used.

Open Questions

While this change is just a bug fix, it still has the potential to break customer traces. Therefore we need to be deliberate with how we release it.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• 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.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-10-06 15:16:48

Comparing candidate commit c05ebe1 in PR branch rey.abolofia/w3c-precedence with baseline commit 9435e66 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.