fix: [2.4] Fix inaccurate partition num metric

[bigsheeper]

The partition number has already been incremented in ChangePartitionState, so there is no need to increment it again in AddPartition.

issue: #37630

pr: #37996

[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.64%. Comparing base (03f01d8) to head (a4ce7af).
Report is 2 commits behind head on 2.4.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.4   #38073      +/-   ##
==========================================
+ Coverage   79.62%   79.64%   +0.02%     
==========================================
  Files        1073     1073              
  Lines      167746   167744       -2     
==========================================
+ Hits       133569   133608      +39     
+ Misses      29734    29699      -35     
+ Partials     4443     4437       -6     

Files with missing lines	Coverage Δ
internal/rootcoord/meta_table.go	83.67% <ø> (-0.03%)	⬇️

... and 30 files with indirect coverage changes

[czs007]

/approve
/lgtm

[czs007]

/run-cpu-e2e

[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[czs007]

/approve
/lgtm

[sre-ci-robot]

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