feat: implement stdvar_over_time function

[haohuaijin]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

I implement the stdvar_over_time function in PromQL and add necessary unit test.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#1229

[codecov]

Codecov Report

Merging #1291 (3dee37b) into develop (6a05f61) will decrease coverage by 0.40%.
The diff coverage is 87.64%.

@@             Coverage Diff             @@
##           develop    #1291      +/-   ##
===========================================
- Coverage    85.88%   85.49%   -0.40%     
===========================================
  Files          500      501       +1     
  Lines        75756    75907     +151     
===========================================
- Hits         65066    64895     -171     
- Misses       10690    11012     +322     

[waynexia]

Sorry for the late reply. It looks very neat!

The same as #1289 (comment):

As stddev is relatively more complicated than other aggr_over_time fns, we need to add more test cases. Actually I found two cases from prometheus

Do you mind adding these two cases to your test as well? And you may also need the this function, let's wait a while before #1289 merged!

[haohuaijin]

@waynexia Thank for your reply.
I have added those two tests. And my implement based on Datafusion implement, it seems that I don't need this function.

[haohuaijin]

Hi, @waynexia
I already resolved the conflict and add stdvar_over_time to functions.rs.

[waynexia]

Great work, thanks for this!