Update relay to support unsampled metrics in Sentry

[mitsuhiko]

It is likely that the volume of unsampled metric datapoints coming from Sentry itself is too high for the existing ingestion relay parser.

Tasks

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1. ref(metrics): Improve distribution value performance #2483
   +0
   
2. Speed up statsd parsing (redundant split/char scans)
3. Avoid redundant split/merge when roundtripping buckets through project cache #2517
   maintenance onboarding +2
   

[jan-auer]

The remaining optimization task is not a critical blocker and has been filed for follow-up.