Bug: mergeHistogram produces non-deterministic bucket order causing flaky tests

[Shvejan]

The mergeHistogram function builds buckets from a Go map without sorting them, causing non-deterministic order. Prometheus client library explicitly
sorts buckets after building from maps and assumes sorted order
for binary search. This causes TestMergeHistogram to fail
intermittently when map iteration produces different orders.

Fix: Add sort.Slice after bucket construction in mergeHistogram(), matching Prometheus's approach.