normalizer: expand "header tag" normalization

[mtoffl01]

What does this PR do?

Drops any header tag input that could result in header or tag of the empty string
Expands testing and logging for header tag normalization

Motivation

AIT-8571
RFC

Reviewer's Checklist

Once merged, we'll have to update this system test: https://github.com/DataDog/system-tests/blob/main/manifests/golang.yml#L405

• 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 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: 2024-02-08 21:16:32

Comparing candidate commit a520750 in PR branch mtoff/fix_header_tags with baseline commit 882cddb in branch main.

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