contrib: header_tags support on julienschmidt/httprouter

[mtoffl01]

What does this PR do?

Implements global and integration-level support for dd_trace_header_tags on the julienschmidt/httprouter integration

Motivation

Fixes #2251

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.
• [N/A] 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: 2023-11-02 19:08:16

Comparing candidate commit 3ab24bf in PR branch mtoff/httprouter-dd_trace_headers with baseline commit d082416 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.