ddtrace/tracer: fix traceparent length check

[ahmed-mez]

What does this PR do?

Align the W3C trace context parsing with the RFC:

• traceparent header with version > 0 can be longer than 55 characters if str[55]=="-"
• The length of traceparent header with version == 0 has to be 55

Motivation

Fixes test_headers_tracecontext.py::test_traceparent_version_0xcc in system tests.

Describe how to test/QA your changes

test_headers_tracecontext.py::test_traceparent_version_0xcc passes in system-tests

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 5f7afeb in PR branch ahmed/fix-traceparent with baseline commit 53225b6 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 18 metrics, 0 unstable metrics.