api: fix GetHotStores

[rleungx]

What problem does this PR solve?

Currently, GetHotStores will return the same value for KeysRead and KeysWritten.

What is changed and how it works?

This PR fixes this problem and improves the test.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1487 into master will increase coverage by 0.16%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1487      +/-   ##
==========================================
+ Coverage    67.2%   67.37%   +0.16%     
==========================================
  Files         158      158              
  Lines       15385    15385              
==========================================
+ Hits        10339    10365      +26     
+ Misses       4107     4084      -23     
+ Partials      939      936       -3

Impacted Files	Coverage Δ
server/schedule/mockcluster.go	83.85% <0%> (ø)	⬆️
server/api/hot_status.go	100% <100%> (ø)	⬆️
pkg/metricutil/metricutil.go	90.62% <0%> (-6.25%)	⬇️
server/util.go	70.27% <0%> (-2.71%)	⬇️
server/leader.go	81.51% <0%> (+0.42%)	⬆️
server/option.go	91.25% <0%> (+0.83%)	⬆️
server/cluster_info.go	86.04% <0%> (+0.83%)	⬆️
server/handler.go	55.87% <0%> (+1.3%)	⬆️
server/core/kv.go	76.81% <0%> (+1.44%)	⬆️
server/region_syncer/server.go	83.62% <0%> (+1.72%)	⬆️
... and 4 more

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[hicqu]

LGTM.