*: add label distribution metrics

[nolouch]

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

What problem does this PR solve?

add label distribution metrics.

What is changed and how it works?

more convenient to get the distribution information of the cluster

Check List

Tests

• Unit test

Related changes

• Need to update the tidb-ansible repository

[codecov-io]

Codecov Report

Merging #1402 into master will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1402      +/-   ##
==========================================
+ Coverage   75.45%   75.46%   +0.01%     
==========================================
  Files         135      135              
  Lines       13068    13078      +10     
==========================================
+ Hits         9860     9869       +9     
+ Misses       2315     2311       -4     
- Partials      893      898       +5

Impacted Files	Coverage Δ
server/metrics.go	100% <100%> (ø)	⬆️
server/store_statistics.go	91.86% <84.61%> (+2.25%)	⬆️
server/api/redirector.go	78.94% <0%> (-5.27%)	⬇️
server/namespace_cluster.go	84.41% <0%> (-3.9%)	⬇️
server/etcd_kv.go	74.54% <0%> (-3.64%)	⬇️
server/core/kv.go	77.53% <0%> (-1.45%)	⬇️
server/leader.go	81.54% <0%> (-0.86%)	⬇️
server/grpc_service.go	53.17% <0%> (-0.49%)	⬇️
server/option.go	88.51% <0%> (ø)	⬆️
client/client.go	68.57% <0%> (+0.47%)	⬆️
... and 5 more

---

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[siddontang]

LGTM

[huachaohuang]

Why does the Jenkins unit test always fail?

[rleungx]

@huachaohuang jenkins-ci-pd/unit-test is our old CI, which is not stable. And this week, I spent some time to make PD's tests run in parallel in idc-jenkins-ci/build. You can see the test result on here. But for now it will run these two CI at the same time. So the old CI could be failed sometimes. Maybe we can switch to idc-jenkins-ci/build which is more stable in the next week.

[huachaohuang]

OK, thanks.

[huachaohuang]

LGTM