organizations:custom-metrics ingestion (ingestion) and organizations:custom-metrics + organizations:ddm-ui (frontend) are the flags

Aims to unblock getsentry/sentry#57907 and get more reliable web vitals data collection. Stretch goal get better metrics emitted from bundler plugins/sentry wizard.

getsentry/sentry-python#2385 and https://github.com/getsentry/sentry-python/blob/master/sentry_sdk/metrics.py