feat(mobile-ui): Add slow, frozen, and total frames metrics

[narsaynorath]

Adds gauge metrics for slow, frozen, and total frames metrics for the Mobile UI Starfish module. The tags I've chosen are common and already applied to other mobile conditions.

The data source for these metrics come from the data property on spans, which I've extracted into measurements on spans.

[iker-barriocanal]

Transactions have these values are measurements (see test). Spans also have measurements, are these values available in there? If not, can we reuse them?

[narsaynorath]

Transactions have these values are measurements (see test). Spans also have measurements, are these values available in there? If not, can we reuse them?

@iker-barriocanal We do have them there, but the problem is that these values are associated with specific spans and we can't really propagate them from the transaction down. And in the current implementation they're sent in the span data property and not measurements.

I see there's a method for extracting values from span data to span measurements here, would it be preferable if I pulled these values into that data structure?

[jjbayer]

Spans also have measurements, are these values available in there? If not, can we reuse them?

I see there's a method for extracting values from span data to span measurements here, would it be preferable if I pulled these values into that data structure?

The difference between transaction measurements and span.measurements is that span measurements are not converted to metrics automatically, so even as a measurement we would need to define a metric in defaults.rs. However, I do believe populating the measurements data structure makes sense, because it will put the numbers into the indexed spans dataset as well.

TL;DR yes, I would populate span.measurements as we do for http.decoded_response_content_length, etc.

[jjbayer]

@narsaynorath could you please update the PR description before merging? Distributions -> gauges, plus maybe mention measurements. Thanks!