feat(apm): add referrer hostname as span tag

[cgcote]

What does this PR do?

Parses referrer hostname from http header when presents and adds it to span tags as http.referrer_hostname.

Motivation

This will be used to identify frontend clients without leaking the PII which can be contained in the entire referrer.

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.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

v2-dev PR: https://github.com/DataDog/dd-trace-go/pull/3343/files

[datadog-datadog-prod-us1]

Datadog Report

Branch report: charles.cote/referrer_host
Commit report: 84679c5
Test service: dd-trace-go

✅ 0 Failed, 5513 Passed, 72 Skipped, 10m 4s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-02 18:04:11

Comparing candidate commit a346d0f in PR branch charles.cote/referrer_host with baseline commit 8819cd7 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 61 metrics, 1 unstable metrics.

[darccio]

Closing this in favour of #3343