fix: align linear_regression to PromQL's behavior

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

• Fix required input param type of predict_linear
• Fix wrong operator priority
• Fix wrong param to linear_regression

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)

• Tracking Issue: Improve PromQL compliance #1042

[codecov]

Codecov Report

Merging #2879 (34c603c) into develop (b3ffe5c) will decrease coverage by 0.54%.
Report is 3 commits behind head on develop.
The diff coverage is 96.22%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2879      +/-   ##
===========================================
- Coverage    84.86%   84.33%   -0.54%     
===========================================
  Files          745      749       +4     
  Lines       117222   117939     +717     
===========================================
- Hits         99482    99465      -17     
- Misses       17740    18474     +734