This moves the minimetrics code from sentry to the SDK.

Todo:

• move and adapt tests
• support unicode values for tag values
• implement gauges as gauges
• add interface to emit internal metrics for deployment

Changes compared to original minimetrics:

• client is gone
• transport and encoder were replaced by a simple function
• types were changed to better align with the sdk style
• timing is implemented as a context manager, what was timing is now distribution
• release, environment and transaction are attached

Fixes getsentry/sentry#55645