[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 (postponed for now)
• Tag normalization
• Docs
• Onboarding docs

Question:

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

Document(s)

Develop Specs
SDK Develop Specs
Product Specs

[kahest]

We also need to add Metrics setup docs to in-product onboarding. See e.g. React Native and Java/Android PRs.

[kahest]

Created a followup issue for onboarding docs - closing this