*_over_time incorrect with subquery

[DanCech]

When using quantile_over_time with a subquery, the results don't match those from prometheus.

Given a series like:

Using a standard query we see that the 7-day 95th percentile figure stays at the correct level after the source series drops off:

Changing it to a subquery by adding :10m the quantile line drops ~8hrs after the raw series.

The plot thickens when using count_over_time:

And prometheus:

With subquery:

vs Prometheus:

[jtlisi]

@codesome Could you take a look at this?

[codesome]

I won't be able to have a look till the end of coming week. I will try though.

[tomwilkie]

I think we assume bound on seek in our iterators, and I think subqueries break these assumptions. Will look for a link.