fix: ETP bug squashes

[hannahkm]

What does this PR do?

Working branch to fix bugs found in ETP (aka v1 trace protocol) while dogfooding. This includes:

• Properly encode metaStructMap values as bytes instead of dropping them.
• Reduced reliance on temporary slices and maps to improve CPU and heap allocation performance.

Motivation

Dogfooding and soon-to-be v1 trace protocol release in v2.7.0.

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.

Unsure? Have a question? Request a review!

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.88%. Comparing base (a4e12a4) to head (17e5ef8).

Files with missing lines	Patch %	Lines
ddtrace/tracer/payload_v1.go	44.44%	43 Missing and 2 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
ddtrace/tracer/payload_v1.go	62.96% <44.44%> (-0.21%)	⬇️

... and 5 files with indirect coverage changes

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