[query] Add support for last_over_time

[ryansammonaiven]

This <aggregation>_over_time function was added in
Prometheus v2.26.0 (changelog, docs).

What this PR does / why we need it:

Fixes #3883.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Add support for `last_over_time`.

Does this PR require updating code package or user-facing documentation?:

NONE

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3884 (d1fb173) into master (9c0ab8e) will increase coverage by 8.4%.
The diff coverage is n/a.

❗ Current head d1fb173 differs from pull request most recent head 3f94f40. Consider uploading reports for the commit 3f94f40 to get more accurate results

@@            Coverage Diff            @@
##           master   #3884      +/-   ##
=========================================
+ Coverage    48.4%   56.8%    +8.4%     
=========================================
  Files         213     552     +339     
  Lines       19364   63291   +43927     
=========================================
+ Hits         9386   36005   +26619     
- Misses       9254   24065   +14811     
- Partials      724    3221    +2497     

Flag	Coverage Δ
aggregator	64.3% <ø> (+1.3%)	⬆️
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.3% <ø> (?)
m3em	46.4% <ø> (ø)
metrics	19.7% <ø> (ø)
msg	74.2% <ø> (ø)

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[fingon]

src/query/test/compatibility/testdata/functions.test might need to have last_over_time uncommented too?

[ryansammonaiven]

src/query/test/compatibility/testdata/functions.test might need to have last_over_time uncommented too?

It appears that when an aggregation returns NaN, it isn't returned, which is why the min_over_time and max_over_time tests were also commented out (referencing "keep NaN"). I uncommented what currently works for all three of these so at least there is some testing like this.

When doing this, I discovered that Prometheus expects the series to keep the metric name (e.g. data) for the last_over_time function, but not the other ones (e.g. min_over_time, max_over_time). This is intentional, and I traced it to this code: https://github.com/prometheus/prometheus/blob/a278ea4b58226c3db271fb78703cd110089cba9b/promql/engine.go#L1292-L1298

Unfortunately, this makes the change a bit more complex than I initially anticipated, as now part of the code that was unconditional before, now needs to be conditional on what function is being executed.

[wesleyk]

@ryansammonaiven thanks for the contribution! Can you update the branch and we'll get it reviewed/merged?

[ryansammonaiven]

@wesleyk thanks for taking a look! I have updated the branch and all checks have passed.

[wesleyk]

@ryansammonaiven looks like you need another update

[ryansammonaiven]

@wesleyk thanks for the approval and merge!

[wesleyk]

@ryansammonaiven we'll be cutting v1.4.0 soon with your change!

[wesleyk]

@ryansammonaiven v1.4.0 is cut now!