metrics, cluster: fix miss heartbeat update

[lhy1024]

What problem does this PR solve?

Close #3199 and close #3558

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Release note

• No release note

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[sonarcloud]

SonarCloud Quality Gate failed.

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
7.5% Duplication

[nolouch]

this lock will become the heartbeat bottleneck.

[codecov]

Codecov Report

Merging #3559 (d76649e) into master (82671c2) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3559      +/-   ##
==========================================
+ Coverage   74.66%   74.75%   +0.09%     
==========================================
  Files         245      245              
  Lines       24090    24111      +21     
==========================================
+ Hits        17986    18024      +38     
+ Misses       4495     4485      -10     
+ Partials     1609     1602       -7     

Flag	Coverage Δ
unittests	74.75% <100.00%> (+0.09%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/cluster/cluster.go	83.81% <100.00%> (+0.88%)	⬆️
server/id/id.go	76.19% <0.00%> (-4.77%)	⬇️
pkg/btree/btree.go	85.30% <0.00%> (-0.97%)	⬇️
server/server.go	72.90% <0.00%> (-0.61%)	⬇️
server/grpc_service.go	47.39% <0.00%> (-0.23%)	⬇️
client/client.go	72.10% <0.00%> (-0.14%)	⬇️
server/statistics/metrics.go	100.00% <0.00%> (ø)
server/tso/allocator_manager.go	78.07% <0.00%> (+0.16%)	⬆️
server/statistics/region_collection.go	97.41% <0.00%> (+0.33%)	⬆️
... and 5 more

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