ddtrace/tracer: fixed propagator not updating the tracestate header a…

[dianashevchenko]

What does this PR do?

Fixes a bug where propagator doesn't update the tracestate header after changing sampling decision on the span.
Also cleans up the tests in regards to that.

Motivation

Describe how to test/QA your changes

• Extract context (pCtx) from header ( verify it's as expected )
• Create a span with ChildOf(pctx) option
• Change priority on the span to a different value.
• Inject span context into an empty headers map and verify that the tracestate is updated

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 152c599 in PR branch shevchenko/w3c-updated with baseline commit e1d94a7 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.