Perf return session length return type

[visumickey]

Fixes #8691

FPRSessionDetails contains the details about the current session. Whenever a trace is created, they take a copy of the session. But the session copy was not taking in the sessionStartTime which could lead to a situation where a check on the session expiration duration could cause a crash. The fix is to make sure the copy contains the same sessionStart time as the original session details object.

[google-oss-bot]

Coverage Report

Affected SDKs

• FirebasePerformance-iOS-FirebasePerformance.framework

  SDK overall coverage changed from 91.86% (9d998a7) to 91.86% (5fe94c3) by +0.00%.

  Filename	Base (9d998a7)	Head (5fe94c3)	Diff
  FPRSessionDetails.m	92.59%	92.86%	+0.26%

Test Logs

• Base (9d998a70): https://github.com/firebase/firebase-ios-sdk/actions/runs/1417899123
• Head (5fe94c3e): https://github.com/firebase/firebase-ios-sdk/actions/runs/1418237652

[paulb777]

Thanks. Fix trailing whitespace issue please.

[leotianlizhan]

LGTM!