Metric cassandra_storage_hints_in_progress must be gauge, not counter

[smarsching]

The cassandra_storage_hints_in_progress is currently exposed as a metric of type counter.

This is incorrect, because according to the specification of Prometheus metric types, a counter must only ever increase and never decrease (except being reset to zero on restart). However, the “hints in progress” metric will decrease whenever hints have been successfully processed.

Therefore, this metrics must be exposed as a gauge instead of a counter.