RUM-1053 Make Logger Sendable for Swift 6 concurrency

[ncreated]

What and why?

📦 This PR prepares DatadogLogs for compatibility with apps that enable "complete" concurrency checking in Swift 6.

I identified a potential data-race safety issue when passing instances of LoggerProtocol across isolation boundaries, such as to different threads:

import DatadogLogs

let logger = Logger.create(with: .init())

DispatchQueue.global(qos: .userInitiated).async {
    logger.debug("foo") 
    // ^ ❌ Error: Capture of 'logger' with non-sendable type 'any LoggerProtocol' in a `@Sendable` closure
}

How?

This issue is resolved by declaring Sendable conformance for LoggerProtocol:

A conformance to Sendable means the given type is thread safe, and values of the type can be shared across arbitrary isolation domains without introducing a risk of data races.

Since Xcode 15 uses SWIFT_STRICT_CONCURRENCY: minimal, which checks only explicit Sendable conformances, this:

- public protocol LoggerProtocol {
+ public protocol LoggerProtocol: Sendable {

resulted in concurrency warnings elsewhere.

Instead of using @unchecked Sendable for RemoteLogger, ConsoleLogger, and some test types, I chose to address concurrency warnings properly, making RemoteLogger and ConsoleLogger fully checked Sendable types 👌.

This required several major changes:

• Followed RUM-3462 refactor: Make FeatureScope always available #1744 and replaced the mutable core reference in RemoteLogger with an immutable reference to FeatureScope.
• Made FeatureScope sendable, to facilitate similar work in other modules.
• Reworked how global Logs attributes are passed to RemoteLogger after removing the core reference.
• Introduced SynchronizedAttributes and SynchronizedTags to ensure RemoteLogger has only Sendable members.
• Added test coverage for Logger tags, which was previously missing.

Smaller adjustments were made as a consequence of spreading Sendable conformance. For example, marking BacktraceReporter as Sendable required resolving issues in Crash Reporting tests, as this type is shared through DatadogInternal.

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

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ncreated/RUM-1053/make-logs-api-concurrency-safe
Commit report: a3d8423
Test service: dd-sdk-ios

✅ 0 Failed, 1880 Passed, 0 Skipped, 1m 16.57s Total Time
🔻 Test Sessions change in coverage: 2 decreased, 5 increased, 1 no change

🔻 Code Coverage Decreases vs Default Branch (2)

• test DatadogRUMTests iOS 81.15% (-0.02%) - Details
• test DatadogCoreTests iOS 72.74% (-0.01%) - Details

[ncreated]

💡 not needed anymore after we dropped iOS 11

[maxep]

/question We are not using the ReadWriteLock as a property wrapper in multiple places, is it because it would require it to be a var? Can't this be solved by making ReadWriteLock Sendable?

[ncreated]

The ReadWriteLock is Sendable already - it was done in this PR. And yes, if using property wrapper, this will become var, raising concurrency issue. I agree with what was discussed here - property wrappers become quite unusable with strict concurrency.

[maxep]

Yes, makes sense 👍

[maxep]

LGTM 👍