fix: sort unstable HTTP result in label values query

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

sort the unstable field list to make result stable

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.66%. Comparing base (36c41d1) to head (3245c73).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3920      +/-   ##
==========================================
- Coverage   85.96%   85.66%   -0.31%     
==========================================
  Files         962      962              
  Lines      164308   164308              
==========================================
- Hits       141252   140749     -503     
- Misses      23056    23559     +503