[ruby] Add metrics

[sl0thentr0py]

https://github.com/getsentry/team-sdks/issues/36

### Tasks
- [ ] https://github.com/getsentry/sentry-ruby/pull/2247
- [ ] timing function
- [ ] callback - before_emit
- [ ] local aggregator on span
- [ ] force flush logic based on weights and thread event
- [ ] code locations
- [ ] units