Add support for Sentry Metrics

[smeubank]

So devs working in Unreal can also take adavantage of Sentry's new Metrics product, the sentry-native SDK will need to finally support Metrics as well. This is the final piece in the puzzle.

Once this is done, we should ensure Unrealy supports based on it's hybrid SDK dependencies, and update the platform SDK docs.

getsentry/sentry-native#953

[smeubank]

moving this to backlog, since changes to the metrics product and API upstream, will return when status and prio changes

[bruno-garcia]

rip