RUMM-2503 Support rulePsr for APM traffic ingestion

[ncreated]

What and why?

📦 This PR sets the rulePsr attribute in RUM resource with the trace sample rate configured in the SDK. This is to provide users with visibility into what APM settings are configured at the SDK level and how much of the traffic is ingested.

The value gets displayed on the APM Ingestion Control page:

To support passing this value from cross-platform SDKs, _dd.rule_psr internal attribute is implemented - as counterpart of recent Android proposal: DataDog/dd-sdk-android#1092.

How?

Trace sample rate value is passed from URLSessionInterceptor all the way down to RUMResourceScope. This is achieved with existing RUMSpanContext transported in "start resource" command.

Cross-platform SDKs should use _dd.rule_psr attribute (passed in any resource-related API, e.g. "stop resource"):

/// Trace sample rate applied to RUM resources created by cross platform SDK.
/// We send cross-platform SDK's sample rate within RUM resource in order to provide accurate visibility into what settings are
/// configured at the SDK level. This gets displayed on APM's traffic ingestion control page.
/// Expects `Double` value between `0.0` and `1.0`.
static let rulePSR = "_dd.rule_psr"

🏕️ I also made small cleanup in RUMResourceScopeTests by removing unnecessary configuration in some of them. The new behaviour is now asserted with 2 new tests - instead of bloating configuration and assertion in previous ones.

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

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ncreated/RUMM-2503-support-rule-PSR-for-APM-ingestion-control
Commit report: a39925e

✅ dd-sdk-ios 0 Failed, 0 New Flaky, 959 Passed, 0 Skipped, 1m21.57s Wall Time

[ncreated]

I wish we could call it better (rulePSR is pretty ambiguous), but I decided to stick with the lingo proposed in rum-event-format and in Android SDK to make it easier for cross-platform.

[ncreated]

Bad JIRA reference, I'm closing this PR in favour of #1029