feat: add process tags to exposed metadata

[raphaelgavache]

What does this PR do?

Add the collection of process tags in dd-trace-go metadata memfd

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.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• 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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-09-10 16:36:16

Comparing candidate commit 1b833c7 in PR branch raphael/process_tags_in_memfd with baseline commit e7e5c7f in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.