chore(w3c): ensure last parent id is never set to 16 zeros

[mabdinur]

What does this PR do?

Currently when w3c tracestate headers are extracted by Datadog W3C Propagators:

1. The p value in the tracestate is stored in SpanContext.reparentID.
2. If the p value is not found in the tracestate then SpanContext.reparentID (which will be used to set Span.meta["_dd.parent_id"]) is set to 0000000000000000.

The second condition is no longer required. Datadog internal services no longer handle the case where Span.meta["_dd.parent_id"] == 0000000000000000. 0000000000000000 is treated as an invalid value and ignored. We should avoid setting this value in client libraries.

Motivation

• Simplify the extraction logic for w3c tracecontext headers.
• Avoids unnecessary tags to the Datadog agent.

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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-07 18:48:45

Comparing candidate commit 3221f07 in PR branch munir/remove-16-zeros-w3c with baseline commit 6e9ec58 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 58 metrics, 0 unstable metrics.

scenario:BenchmarkInjectW3C-24

• 🟥 execution_time [+110.220ns; +147.380ns] or [+2.682%; +3.586%]

[darccio]

LGTM