[react-native] Add support for Sentry Metrics

[smeubank]

Description

Metrics! Sentry will support Metrics finally! First we need all of our SDKs to support Metrics, including react-native

Open point:

How much of this can the react-native SDK inherit from the JS SDK?

Document(s)

Develop Specs
SDK Develop Specs
Product Specs

### Tasks
- [x] Re-export the metric APIs
- [x] Add an example to the sample applications
- [x] Code snippet to changelog

[krystofwoldrich]

I've tested the browser API in our Expo and RN sample apps, both send data to Sentry. So we only need to re-export the API in RN SDK.