ddtrace/tracer: increment droppedP0Traces iff it's not going to be sent

[darccio]

What does this PR do?

Reduce reported dropped traces by increasing the counter only when the chunk won't be sent.

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: 2024-05-29 08:17:33

Comparing candidate commit 650ee60 in PR branch dario.castane/report-drop-only-iff-not-willsend with baseline commit b913c87 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 47 metrics, 0 unstable metrics.