Add unit test for DataNodeNum metrics value

[congqixia]

See also #25984
/kind improvement

[sre-ci-robot]

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[codecov]

Codecov Report

Merging #25985 (46cf41a) into master (eaf53c7) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #25985      +/-   ##
==========================================
- Coverage   82.27%   82.25%   -0.02%     
==========================================
  Files         803      803              
  Lines      106888   106889       +1     
==========================================
- Hits        87939    87919      -20     
- Misses      15861    15880      +19     
- Partials     3088     3090       +2     

see 14 files with indirect coverage changes

[jiaoew1991]

/lgtm