[Dart] Add support for Sentry Metrics

[kahest]

Description

Metrics! Sentry will support Metrics finally! First we need all of our SDKs to support Metrics, including Dart SDK (flutter) and dependent SDKs

Requirements:

• [ ] Matrics API
• [ ] Metrics Summary
• [ ] Code location
• [ ] Tag normalization

Question:

How much of this can the Android SDK inherit if added in Java?

Document(s)

Develop Specs SDK Develop Specs Product Specs