[cocoa] Add support for Sentry Metrics

[kahest]

Description

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

Requirements:

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

Document(s)

Develop Specs SDK Develop Specs Product Specs

### Tasks
- [ ] https://github.com/getsentry/sentry-cocoa/pull/3676