statistics: multidimensional hot peer cache (#2140)

[sre-bot]

cherry-pick #2140 to release-3.1

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What problem does this PR solve?

First step of #2139 . The hot peer cache is not working with multidimensional load.

What is changed and how it works?

Modify the TopN to support multidimensional load.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2244 into release-3.1 will increase coverage by 0.04%.
The diff coverage is 87.31%.

@@               Coverage Diff               @@
##           release-3.1    #2244      +/-   ##
===============================================
+ Coverage        74.86%   74.91%   +0.04%     
===============================================
  Files              196      196              
  Lines            20179    20222      +43     
===============================================
+ Hits             15108    15150      +42     
+ Misses            3922     3916       -6     
- Partials          1149     1156       +7     

Impacted Files	Coverage Δ
server/api/trend.go	68.75% <0.00%> (ø)
server/statistics/util.go	100.00% <ø> (+3.84%)	⬆️
server/statistics/hot_peer_cache.go	85.18% <64.51%> (-3.35%)	⬇️
server/schedulers/hot_region.go	85.46% <80.00%> (ø)
server/statistics/topn.go	97.92% <94.80%> (-2.08%)	⬇️
server/statistics/hot_peer.go	100.00% <100.00%> (ø)
server/region_syncer/client.go	79.66% <0.00%> (-6.78%)	⬇️
server/heartbeat_streams.go	67.00% <0.00%> (-6.00%)	⬇️
server/tso/tso.go	77.37% <0.00%> (-2.92%)	⬇️
server/server.go	64.15% <0.00%> (-0.43%)	⬇️
... and 7 more

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[nolouch]

/run-all-tests

[lhy1024]

Can we directly cherry-pick this pr to 3.1 without others

[disksing]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for 2252

[sre-bot]

/run-all-tests