contrib/net/http: harmonize benchmarks for v2

[darccio]

What does this PR do?

BenchmarkHttpServeTrace used the mocktracer instead of the real tracer.

Motivation

As in v2 we prefer to use the real tracer, although not completely applied, we found that we were measuring different implementations.

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: 2024-03-19 08:24:59

Comparing candidate commit f0df2d9 in PR branch dario.castane/AIT-9399/harmonize-benchmarks with baseline commit b4da5f8 in branch main.

Found 0 performance improvements and 3 performance regressions! Performance is the same for 37 metrics, 1 unstable metrics.

scenario:BenchmarkHttpServeTrace-24

• 🟥 allocated_mem [+4.453KB; +4.475KB] or [+89.553%; +90.003%]
• 🟥 allocations [+30; +30] or [+61.224%; +61.224%]
• 🟥 execution_time [+6.351µs; +6.454µs] or [+62.950%; +63.980%]

[darccio]

Using the real tracer instead of mocktracer results in higher metric values, as expected.

Thanks Diana!