tracer: enable 128-bit TraceID generation by default

[katiehockman]

What does this PR do?

Changes DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED default to true

Note that DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED default does not change, and it remains false.

Motivation

Better support W3C standards for TraceID

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• 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.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!
AIT-8848

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-11-10 10:04:50

Comparing candidate commit 07e257e in PR branch katie.hockman/128-default with baseline commit 9d39644 in branch main.

Found 0 performance improvements and 8 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

scenario:BenchmarkOTelApiWithCustomTags/datadog_otel_api-24

• 🟥 allocated_mem [+93 bytes; +102 bytes] or [+2.651%; +2.913%]
• 🟥 allocations [+2; +2] or [+7.143%; +7.143%]
• 🟥 execution_time [+125.363ns; +195.437ns] or [+2.498%; +3.895%]

scenario:BenchmarkOTelApiWithCustomTags/otel_api-24

• 🟥 allocated_mem [+99 bytes; +110 bytes] or [+2.640%; +2.934%]
• 🟥 allocations [+2; +2] or [+6.452%; +6.452%]
• 🟥 execution_time [+190.315ns; +266.085ns] or [+3.279%; +4.584%]

scenario:BenchmarkTracerAddSpans-24

• 🟥 allocations [+2; +2] or [+8.696%; +8.696%]
• 🟥 execution_time [+114.939ns; +155.461ns] or [+2.990%; +4.044%]

[darccio]

@katiehockman I think the other test that fails can be fixed like this:

func TestSpanWithoutNewRoot(t *testing.T) {
	assert := assert.New(t)
	otel.SetTracerProvider(NewTracerProvider())
	tr := otel.Tracer("")

	parent, ddCtx := tracer.StartSpanFromContext(context.Background(), "otel.child")
	_, child := tr.Start(ddCtx, "otel.child")
	// Change the rest of the method with the following lines:
	parentCtxW3C := parent.Context().(ddtrace.SpanContextW3C)
	assert.Equal(parentCtxW3C.TraceID128Bytes(), [16]byte(child.SpanContext().TraceID()))
}

[katiehockman]

Thanks @darccio !!