ddtrace: tracer: wip support for meta_struct field in span

[eliottness]

What does this PR do?

This PR adds the ability to send metadata in spans via the field meta_struct.

Motivation

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 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: 2024-02-16 14:08:49

Comparing candidate commit e47a415 in PR branch eliott.bouhana/meta-struct with baseline commit c86c219 in branch main.

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