internal/globalconfig: make statsTags un-changeable from outside of the globalconfig package

[mtoffl01]

What does this PR do?

• globalconfig.SetStatsTags(tags []string) now makes a copy of tags before setting it as the value of cfg.statsTags
• globalconfig.StatsTags() now returns a copy of cfg.statsTags

Motivation

Failure to perform the copying specified above resulted in pollution of the globalconfig statsTags.
#2836

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-03 18:17:12

Comparing candidate commit ec94a25 in PR branch mtoff/global-statstags with baseline commit 2c85bdc in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 58 metrics, 0 unstable metrics.

scenario:BenchmarkSetTagString-24

• 🟥 execution_time [+2.661ns; +4.759ns] or [+2.376%; +4.249%]

[rolandhawk]

@mtoffl01 how is this PR going on? 🙏

[darccio]

LGTM

@rolandhawk It'll be merged after my approval, and it'll get released during September.