Fix off-by-one error in funcHistogramQuantile / ensureMonotonic

[linasm]

Signed-off-by: Linas Medziunas linas.medziunas@gmail.com

Fixes an off-by-one error in ensureMonotonic (that is used in histogram quantiles calculation):

• original implementation was unintentionally iterating from 0 to len(buckets) - 2, thus not fixing the last bucket value
• fixed implementation iterates from 1 to len(buckets) - 1 as intended

[brian-brazil]

Thanks, could we have tests around each of the non-monotonic buckets?

[roidelapluie]

Working on tsdb test failure at #7394

[roidelapluie]

When you add the tests Brian asked, would you please rebase on top of master? Thanks!

[linasm]

@brian-brazil it is not possible to hit the non-monotonic buckets with binary search directly (because their value is made equal to the value of the preceding bucket). So I have altered the test data to alternate the buckets (increase -> decrease -> increase -> ...) and added test queries to hit every increase -> decrease pair.
Let me know if this is similar to what you were asking for (I'm not 100% sure).

@roidelapluie thanks for fixing the master so quickly.

[brian-brazil]

Thanks!