tracer: allow x-datadog headers to pass as header tags

[mtoffl01]

What does this PR do?

Allows x-datadog headers to be set as tags on http integrations with the header_tags feature

Motivation

Previously, the Go tracer would drop any headers prefixed with "x-datadog". Now that header_tags need to be explicitly included, there is no reason to drop x-datadog headers if they've been specified.
Allowing these headers as tags also assists with troubleshooting trace context propagation issues, see: Document

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!

[darccio]

LGTM

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-07-11 14:28:57

Comparing candidate commit 9c358af in PR branch mtoff/header-tags with baseline commit e8dc974 in branch main.

Found 2 performance improvements and 0 performance regressions! Performance is the same for 44 metrics, 1 unstable metrics.

scenario:BenchmarkHttpServeTrace-24

• 🟩 allocations [-2; -2] or [-2.532%; -2.532%]
• 🟩 execution_time [-491.671ns; -403.129ns] or [-2.980%; -2.443%]