RUMM-1278 Screen Refresh Rate vitals

[buranmert]

What and why?

We will be reading the refresh rate during app's lifetime in order to detect UI hitches.

How?

iOS renders UI in the main run loop, this is a RunLoop instance. We want to be notified when a new frame is drawn.
CADisplayLink is a special timer type which listens to frame paints in a given run loop.
We add a CADisplayLink instance to RunLoop.main so that it calls our method every time a new frame is drawn.

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

[ncreated]

I left thread synchronization question as IMO there is an unnecessary penalty on the main thread and I don't clearly see how we will further read these value form secondary thread in RUM scope.

@xgouchet how do you synchronize publishing this vital from main thread and reading it from RUM scope thread on Android?

[xgouchet]

I left thread synchronization question as IMO there is an unnecessary penalty on the main thread and I don't clearly see how we will further read these value form secondary thread in RUM scope.

@xgouchet how do you synchronize publishing this vital from main thread and reading it from RUM scope thread on Android?

On Android the RUM View Scope doesn't read from it. What happens is the view gets notified when a new aggregation is available and stores its value locally, and when it sends the event it sends it with the latest local value it knows

[ncreated]

Looks very good 👌