fix(options): Don't override c.httpClient if it is set via the options

[BaptisteFoy]

What does this PR do?

Prevents overrides to c.httpClient if it has been set by customers; even if the agent URL points to a unix socket

Motivation

For my software to work I had to update the HTTP client of my tracer. Using the latest agent and tracer and letting other settings as default, the HTTP client override didn't work.

This is because:

1. Newer agent creates the socket by default ([APM] Add a default UDS listeners for traces and stats datadog-agent#26969)
2. As the documentation states, if the socket file exists and no env var is set, the socket will be used as the agent URL
3. If the socket is used as the agent URL, we override c.httpClient regardless of whether it has been previously set

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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-07 17:54:20

Comparing candidate commit 466bf66 in PR branch baptiste.foy/dont-override-clients with baseline commit bebced4 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 59 metrics, 0 unstable metrics.

[mtoffl01]

Great catch!