[Metrics]: Add Metrics to buffer

[https://develop.sentry.dev/sdk/telemetry/metrics/#buffering](https://develop.sentry.dev/sdk/telemetry/metrics/#buffering)

## Now
For now, we’ll use an in-memory buffer. 


## Future
Switch to using the native buffer implementations on iOS and Android (once they are implemented) so that data can be cached on those platforms as well - all other platforms will continue to use the in-memory buffer.