RUMM-2166 Add tracing sampling rate

[maxep]

What and why?

Offer a configuration option to set the tracing sampling rate. Based on this sampling rate, new network requests will have the x-datadog-sampling-priority header set to 0 (or 1 accordingly), and the trace_id and span_id won’t (or will accordingly) be added to the created RUM Resource.

Note: The x-datadog-sampled is now removed.

When sampled out, a RUM resource appears with no trace:

How?

The HTTPHeadersWriter now expect a samplingRate value at initialization. If the rate is 0, the trace id and span id are not propagated to APM nor RUM when instrumenting URLSession. The x-datadog-sampling-priority is set accordingly when a request is intercepted.

The samplingRate is configured by the FeaturesConfiguration.URLSessionAutoInstrumentation and injected to URLSessionInterceptor at initialization.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests
• Run integration tests
• Run smoke tests

[maxep]

I'm targeting the release/1.11.0 branch that was pulled from release/1.11.0-beta2

[ncreated]

All good! ✅