Export SamplingPriority method on spanContext [SVLS-3934]

[purple4reina]

What does this PR do?

Exports the SamplingPriority method on the spanContext type.

Motivation

We on the serverless team are working to update the Lambda Extension from doing manual trace context extraction to instead deferring to dd-trace-go for all extraction. This will allow us to be able to support W3C trace context headers.

The problem is that we also need to have access to the sampling priority. We must add this value to the trace chunk before it's processed by the trace agent.

Currently, dd-trace-go offers no way for us to get access to the sampling priority.

Discussion

I am very much open to discussion for how best to get the sampling priority from the SpanContext. I have chosen to simply export the SamplingPriority method because it's the lightest touch creating a small diff. Furthermore, the exported method will not show up in godocs and therefore not be immediately visible to users. One would have to read the code and specifically cast the SpanContext to a new interface to get access to it.

Here's how I'm proposing to use the method:

func getSamplingPriority(sc ddtrace.SpanContext) (priority sampler.SamplingPriority) {
	priority = defaultPriority
	if pc, ok := sc.(interface{ SamplingPriority() (int, bool) }); ok && pc != nil {
		if p, ok := pc.SamplingPriority(); ok {
			priority = sampler.SamplingPriority(p)
		}
	}
	return
}

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-10-25 18:01:58

Comparing candidate commit cb0edc7 in PR branch rey.abolofia/sampling-priority with baseline commit a95d449 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.

[darccio]

I see no reason to not export this function. LGTM 👍