[Performance] Fix missing network traces for Swift async/await URLSession requests on iOS 16+ (#11861)

[JesusRojass]

Discussion

Fixes #11861

Firebase Performance instruments NSURLSession by swizzling ObjC methods on session delegate
classes and task creation methods. This works well for traditional ObjC style request patterns
(completion handlers, delegate callbacks), but Swift async/await URLSession methods like
data(for:), download(for:), bytes(for:) are pure Swift extensions on URLSession with
no ObjC selector, so they are invisible to method swizzling and not traced on iOS 16+.

Changes

Two NSURLSessionTaskDelegate callbacks are now instrumented for both ObjC and async/await
created tasks:

• URLSession:didCreateTask: (iOS 16+): entry point to attach a FPRNetworkTrace to the
  task. Skips if the ObjC task creation swizzle already attached one.

• URLSession:task:didFinishCollectingMetrics: (iOS 10+): exit point to complete and
  remove the trace. Uses NSURLSessionTaskTransactionMetrics for accurate byte counts. The
  traceCompleted guard in FPRNetworkTrace makes this safe to call more than once: for
  completion handler tasks the trace is already gone (early return); for delegate based tasks
  this fires first and didCompleteWithError: becomes a no op.

Both hooks are wired into FPRNSURLSessionDelegate (no user delegate) and
FPRNSURLSessionDelegateInstrument (user delegate present, via class level swizzle and
CopySelector).

Testing

All existing tests pass.

New tests (FPRNSURLSessionInstrumentTest)

• testDelegateURLSessionTaskDidFinishCollectingMetrics: real HTTP request against the
  hermetic test server, verifies the delegate flag is set and the trace removed after response.

• testDelegateURLSessionTaskDidFinishCollectingMetricsCopiedForNonImplementingDelegate:
  verifies CopySelector installs didFinishCollectingMetrics: on delegates that do not
  implement it, ensuring async/await tasks are traced even for sparse delegate implementations.

• testDidCreateTaskDoesNotDoubleTraceObjCCreatedTask: verifies that when didCreateTask:
  fires for an ObjC created task the existing trace is preserved and no duplicate is created.

• testDelegateURLSessionDidCreateTaskCalledOnIOS16: verifies the didCreateTask: hook
  is wired through the class level swizzle and reaches the user delegate on iOS 16+.

Pre-existing test fixes

• testDelegateURLSessionTaskDidCompleteWithError (FPRNSURLSessionInstrumentTest):
  replaced http://nonurl + 5 second DNS spin with stopped local server for a deterministic
  "connection refused" failure.

• testConnectionDidFailWithError (FPRNSURLConnectionInstrumentTest): same fix. The
  new didFinishCollectingMetrics: instrumentation added enough overhead to the shared
  delegate queue to push the DNS timeout past the 2 second window.

API Changes

• No API Changes.

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[JesusRojass]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for Swift async/await network tasks by implementing and instrumenting URLSession:didCreateTask: and URLSession:task:didFinishCollectingMetrics: delegate methods. The changes also include updates to unit tests for more deterministic failure scenarios and new tests for the async/await functionality. Feedback suggests refactoring duplicated logic in the instrumentation methods into common helper functions for improved maintainability. Additionally, it's recommended to remove > 0 checks when assigning responseSize and requestSize from NSURLSessionTaskTransactionMetrics to ensure accuracy and consistency, as these metrics are considered reliable.

[JesusRojass]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for Swift async/await network instrumentation in Firebase Performance. It adds handling for URLSession:didCreateTask: (iOS 16+) and URLSession:task:didFinishCollectingMetrics: (iOS 10+) to capture network traces for tasks where traditional delegate callbacks might be suppressed. The changes include new helper functions, updated instrumentation logic in FPRNSURLSessionDelegateInstrument, and comprehensive unit tests to verify the new behavior and ensure no duplicate traces are created. I have no feedback to provide.

[JesusRojass]

The machine is pleased and so am I

Ready for review @visumickey

[JesusRojass]

@visumickey All CI/CD check should hopefully pass now!

[JesusRojass]

Hello there @tejasd

Wondering if you could help me review this one too please!

[JesusRojass]

@tejasd @mghaznav Tagging for review!