feat: don't map semantic type in metric engine

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Avoid memory problem caused by null fields. By not mapping tag to field in metric engine.

Peak memory drops from ~24GB to ~10GB

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.

Refer to a related PR or issue link (optional)

[fengjiachun]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8485c9a) 85.86% compared to head (0ac984d) 85.17%.
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3243      +/-   ##
==========================================
- Coverage   85.86%   85.17%   -0.69%     
==========================================
  Files         844      856      +12     
  Lines      138631   140066    +1435     
==========================================
+ Hits       119034   119300     +266     
- Misses      19597    20766    +1169