feat(metrics): Log statsd metrics for bucket flushing

[jan-auer]

Introduces new internal metrics to measure the pressure and performance
on the metric bucket flush cycle:

1. metrics.buckets.scan_duration: Time in milliseconds spent scanning
   metric buckets to flush.
2. metrics.buckets.flushed: The total number of metric buckets flushed
   in a cycle across all projects.
3. metrics.buckets.flushed_per_project: The number of metric buckets
   flushed in a cycle for each project.
4. metrics.buckets: The total number of metric buckets in Relay's
   metrics aggregator.

[RaduW]

looks good