Log outcomes for dropped metrics buckets

[jjbayer]

We do not generate any outcomes for dropped buckets:

https://github.com/getsentry/relay/blob/4cb923f0667da94801039ff2cbd99635221dd97b/relay-metrics/src/aggregation.rs#L2006-L2009

Not sure if it even makes sense to submit outcomes at the point where the metrics aggregator shuts down, because this means that the outcomes aggregator is probably also shutting down.