Fix offset finder

[arkbriar]

Fixes #678

The offset finder didn't check the offset value, which incorrectly found an offset of 0 in the query below

max(max_over_time(sum(rate(stream_executor_row_count{namespace=~"default"}[30s] offset 30m))[12h2m27s:30s]))

, and it returns an error of mismatched offsets which is swallowed silently.

With the fix, the query above correctly triggered QueryRange, as showed below.

DEBU[2024-09-10T19:55:38+08:00] http://localhost:8428                         api=QueryRange query="sum(rate(stream_executor_row_count{namespace=~\"default\"}[30s]))" r="{2024-09-08 23:23:30 +0000 UTC 2024-09-10 11:25:38.141 +0000 UTC 30s}"

The request time range is [2024-09-08 23:53:30 +0000 UTC 2024-09-10 11:55:38.141] and the offset 30m was respected.

[arkbriar]

I suddenly realise that sum shouldn't be pushed down since merging sum won't produce a correct result. To correctly handle the sum, promxy has to merge the original time series matrix and calculate itself. However, after I looked into the query_range API, I found that it doesn't support streaming results which makes it impossible to reduce the memory footprint by merging results from servers without materialising the data (i.e., with a merged iterator).

I don't know what to do now. The PR merely makes certain queries being able to be pushed down. But it's fixing a bug upon another and becomes meaningless.

[jacksontj]

Closing this PR to take the convo back to #678 :)