statistic: fix the issue of tombstone for label counter

[nolouch]

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 #2060 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2060      +/-   ##
==========================================
+ Coverage   76.63%   76.72%   +0.09%     
==========================================
  Files         189      189              
  Lines       19231    19233       +2     
==========================================
+ Hits        14737    14757      +20     
+ Misses       3374     3355      -19     
- Partials     1120     1121       +1

Impacted Files	Coverage Δ
server/statistics/store_collection.go	59.25% <100%> (+16.4%)	⬆️
server/kv/etcd_kv.go	72.72% <0%> (-11.69%)	⬇️
server/region_syncer/client.go	78.31% <0%> (-2.41%)	⬇️
server/core/storage.go	71.35% <0%> (-1.05%)	⬇️
server/member/leader.go	78.57% <0%> (-1.03%)	⬇️
server/config/option.go	90.32% <0%> (-0.93%)	⬇️
server/server.go	81.85% <0%> (-0.38%)	⬇️
server/handler.go	50.85% <0%> (+0.42%)	⬆️
server/grpc_service.go	58.96% <0%> (+0.43%)	⬆️
client/client.go	73.44% <0%> (+1.2%)	⬆️
... and 3 more

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[rleungx]

LGTM

[lhy1024]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@nolouch merge failed.

[nolouch]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 in PR #2067

[sre-bot]

cherry pick to release-3.1 in PR #2068