statistic: fix the issue of tombstone for label counter (#2060)

[sre-bot]

cherry-pick #2060 to release-3.0

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Signed-off-by: nolouch nolouch@gmail.com

What problem does this PR solve?

Fix issue #2004. Do not count the tombstone store.

What is changed and how it works?

Do not count the tombstone store.

Check List

• Unit test

[codecov-io]

Codecov Report

Merging #2067 into release-3.0 will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           release-3.0    #2067      +/-   ##
===============================================
- Coverage        69.52%   69.48%   -0.05%     
===============================================
  Files              170      170              
  Lines            16800    16802       +2     
===============================================
- Hits             11681    11675       -6     
- Misses            4145     4151       +6     
- Partials           974      976       +2

Impacted Files	Coverage Δ
server/statistics/store_collection.go	85% <66.66%> (-0.51%)	⬇️
pkg/etcdutil/etcdutil.go	79.06% <0%> (-6.98%)	⬇️
server/util.go	68.15% <0%> (-4.46%)	⬇️
server/tso.go	74.46% <0%> (-2.13%)	⬇️
server/namespace_cluster.go	85.71% <0%> (+3.89%)	⬆️
server/schedulers/random_merge.go	67.5% <0%> (+5%)	⬆️

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[rleungx]

LGTM

[rleungx]

/run-all-tests