Sort upperBound by ascending order in mergeHistogram

[SungJin1212]

I have noticed the TestMergeHistogram is flaky. The reason is mergeHistogram() merged classic histogram buckets via a map[float64]uint64 (bucketMap), since Go map iteration is non-deterministic, the resulting bucket order could vary.
This PR sorts by Bucket.UpperBound in ascending order and keeps the output stable.

Related Log

--- FAIL: TestMergeHistogram (0.00s)
    --- FAIL: TestMergeHistogram/native_+_classic (0.00s)
        metrics_helper_test.go:1562: 
            	Error Trace:	/__w/cortex/cortex/pkg/util/metrics_helper_test.go:1562
            	            				/__w/cortex/cortex/pkg/util/metrics_helper_test.go:1609
            	Error:      	Not equal: 
            	            	expected: 1
            	            	actual  : 5
            	Test:       	TestMergeHistogram/native_+_classic
FAIL

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]