[v2] ddtrace/tracer: tidy up Tracer interface

[darccio]

What does this PR do?

This PR tidies up the Tracer interface for v2. It adds some functions to allow tracers implementing the interface to integrate with our implementation.

Motivation

v2 work was interrupted due to team changes. To reach release candidate quality we need to resolve pending TODOs regarding public API.

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: 2024-03-26 15:51:28

Comparing candidate commit 7c15f9f in PR branch dario.castane/tracer-iface-tidyup with baseline commit 42807ee in branch v2-dev.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.

[knusbaum]

Looks good!

[github-actions]

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