Replace CAS contention histograms in storage proxy stats with
estimated_histogram_with_max<128> and switch metrics/API aggregation to the
new histogram path.
Introduce a dedicated cas_contention_histogram alias and use it for
cas_read_contention and cas_write_contention.
Update API histogram reduction to merge the new histogram type via
estimated_histogram_with_max_merge.
Convert API JSON serialization to explicit offsets/counts using
get_buckets_offsets() and get_buckets_counts().
Export CAS contention metrics with to_metrics_histogram(...) instead of the
legacy get_histogram(1, 8) path for consistent bucket handling.
03d7ab17c9