feat: support querying field column names in Prometheus HTTP API

[waynexia]

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What's changed and what's your intention?

The extended special matcher __field__ is used to specify which field to query in multi-field scenario. This patch adds support for querying the possible values of __field__

It works like

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

Attention: Patch coverage is 80.39216% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 85.33%. Comparing base (cc8d6b1) to head (5496530).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3880      +/-   ##
==========================================
- Coverage   85.63%   85.33%   -0.31%     
==========================================
  Files         952      952              
  Lines      163341   163496     +155     
==========================================
- Hits       139878   139515     -363     
- Misses      23463    23981     +518     

[fengjiachun]

LGTM