server: add tso handle time metrics

[disksing]

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

When pd-client reports slow tso, we don't know if it takes too much time for server to handle the request.

What is changed and how it works?

Add metrics.

Check List

Tests

• Integration test

Related changes

• Need to cherry-pick to the release branch
• Need to update the tidb-ansible repository

[codecov-io]

Codecov Report

Merging #1502 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1502      +/-   ##
==========================================
- Coverage   67.45%   67.36%   -0.09%     
==========================================
  Files         158      158              
  Lines       15518    15521       +3     
==========================================
- Hits        10467    10456      -11     
- Misses       4109     4113       +4     
- Partials      942      952      +10

Impacted Files	Coverage Δ
server/metrics.go	100% <100%> (ø)	⬆️
server/grpc_service.go	50.99% <100%> (+0.21%)	⬆️
server/util.go	70.27% <0%> (-7.44%)	⬇️
server/etcd_kv.go	63.63% <0%> (-7.28%)	⬇️
server/leader.go	78.57% <0%> (-6.31%)	⬇️
server/api/redirector.go	78.94% <0%> (-5.27%)	⬇️
server/handler.go	55.06% <0%> (-0.52%)	⬇️
client/client.go	68.29% <0%> (-0.24%)	⬇️
server/tso.go	72.72% <0%> (+3.4%)	⬆️
server/region_syncer/server.go	81.89% <0%> (+3.44%)	⬆️
... and 5 more

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