Fix: Support custom propagators in startup log

[mtoffl01]

What does this PR do?

Modifies the logic within the startup log for determining values for propagation injectors and extractors.
Fixes #2923

Motivation

The previous logic expected to cast the current propagator to a *chainedPropagator, but that's not always viable, especially in the case that a custom propagator has been registered on the tracer via the WithPropagator StartOption.

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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-15 16:40:25

Comparing candidate commit bb1e793 in PR branch mtoff/fix-startup-log with baseline commit 93311db in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 0 unstable metrics.

scenario:BenchmarkInjectW3C-24

• 🟩 execution_time [-129.873ns; -91.527ns] or [-3.028%; -2.134%]