These allow users to see stats about the events that never got to Sentry, caused by a multitude of reasons (e.g. breadcrumb buffer overflow, sample rate discarding, before_send filtering, internal errors, ...).

This feature is described in detail on the following develop docs page: https://develop.sentry.dev/sdk/telemetry/client-reports/

We can take inspiration from the Java/Android SDK getsentry/sentry-java#1894 (or the other SDKs mentioned on the linked issue)