[part 2] refactor!(telemetry): move telemetry logic from DatadogAgent

[dmehala]

Description

This PR continues the work of refactoring the telemetry module to provide a clear and structured telemetry API. The goal is to make it easier for Datadog engineers to collect telemetry metrics and logs on the usage of our products.

Specifically, this PR moves the telemetry logic out of DatadogAgent and into a the telemetry module.
For reference, Part 1 of this refactoring can be found here: #166.

[pablomartinezbernardo]

nit: Can now remove user_metrics_ from tracer_telemetry.h

[dmehala]

That's a mistake, thank you for raising this. Addressed in b493cee

[dubloom]

This PR introduces a coupling between tracer_telemetry and telemetry which does not seem standard in comparison with the other tracers. Is there any objective or removing the tracer_telemetry later ? I would personally prefer if everything was centralized.

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-04-03 09:49:11

Comparing candidate commit 56ddaaf in PR branch dmehala/telemetry/part-2-move-telemetry-exporter with baseline commit 67ab91f in branch main.

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

scenario:BM_TraceTinyCCSource

• 🟥 execution_time [+1.971ms; +2.179ms] or [+2.644%; +2.923%]

[dmehala]

@dubloom the coupling in temporary. While we aim for standardization and feature parity, only public facing code and API are bound by that requirement.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.57364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 93.38%. Comparing base (67ab91f) to head (56ddaaf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datadog/telemetry/telemetry.cpp	96.73%	3 Missing ⚠️
src/datadog/tracer.cpp	84.61%	2 Missing ⚠️
src/datadog/datadog_agent.cpp	80.00%	1 Missing ⚠️
src/datadog/tracer_config.cpp	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   93.15%   93.38%   +0.22%     
==========================================
  Files          76       78       +2     
  Lines        4471     4487      +16     
==========================================
+ Hits         4165     4190      +25     
+ Misses        306      297       -9     

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