Allow custom instrumentation to collect slow and frozen frames

[ueman]

I would like to be able to use the frames integration on macOS. Currently that's not possible because it depends on native code, which is also not able to provide frame information.

However I could gather frame informations in pure Flutter code, based on #699 . That would also enable that feature on Linux and Windows and it's compatible from Flutter 2.0 onwards.

Therefore, I would like to have a way to somehow achieve this.

[marandaneto]

@ueman we're experimenting with a new public API for adding custom measurements, See getsentry/sentry-python#1359
Not sure if we will make SentryNative flexible enough, but rather give you a proper way to report any measurement based on your own way.

[ueman]

Yeah, but then I would also have to write my own NavigationObserver and so on. The support for custom metrics is pretty exciting.

[philipphofmann]

We will make an abstraction in the API that when you use the custom instrumentation, slow and frozen frames are attached automatically.