fix and cleanup the Content-Length handling in transport

[paulcacheux]

What does this PR do?

This PR cleans up the Content-Length header in the request made to the agent. In the case of civisibility the request is made with a *bytes.Buffer and as indicated in the documentation the content length is computed automatically here (cf doc).

In the case of the default/regular trace transport, the Content-Length was set manually through the header. My investigation from the agent side is that we receive a fair amount of traces from dd-trace-go without Content-Length, so this PR moves the content length back to the actual request field, as suggested in the documentation.

Here is a playground example showing how setting the header manually is not working as expected https://go.dev/play/p/AGdBGGZ7IgY

Motivation

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: 2024-10-29 21:21:37

Comparing candidate commit 5355c5d in PR branch paulcacheux/content-length-cleanup with baseline commit fcda5e2 in branch main.

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