*: make flow-round-by-digit compatible with trace-region-flow

[nolouch]

Signed-off-by: nolouch [email protected]

What problem does this PR solve?

deprecate trace-region-flow

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  situation 1

### old server
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config set trace-region-flow false
Success!
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config show all |grep flow        
"trace-region-flow": "false"

### start new binary
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config show all |grep flow
"flow-round-by-digit": 9223372036854775807
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config set flow-round-by-digit 9
Success!
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config show all |grep flow      
"flow-round-by-digit": 9
### restart new binary
➜  pd git:(remove-trace-flow) ✗ ./bin/pd-ctl config show all |grep flow      
"flow-round-by-digit": 9

situation 2

### old server
➜  pd git:(remove-trace-flow) ./bin/pd-ctl config show all |grep flow
    "trace-region-flow": "true"
### restart with new binary
➜  pd git:(remove-trace-flow) ./bin/pd-ctl config show all |grep flow
    "flow-round-by-digit": 3
### restart with new binary
➜  pd git:(remove-trace-flow) ./bin/pd-ctl config show all |grep flow
    "flow-round-by-digit": 3

Release note

 Deprecate  `trace-region-flow`, and use new config `flow-round-by-digit`

[codecov]

Codecov Report

Merging #3747 (f387675) into master (a9e629d) will increase coverage by 0.09%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master    #3747      +/-   ##
==========================================
+ Coverage   75.10%   75.20%   +0.09%     
==========================================
  Files         244      244              
  Lines       24477    24492      +15     
==========================================
+ Hits        18384    18418      +34     
+ Misses       4472     4460      -12     
+ Partials     1621     1614       -7     

Flag	Coverage Δ
unittests	75.20% <70.58%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
server/config/config.go	74.84% <64.28%> (-0.32%)	⬇️
server/cluster/cluster.go	82.76% <100.00%> (ø)
server/config/persist_options.go	91.40% <100.00%> (+0.03%)	⬆️
server/schedulers/utils.go	93.75% <0.00%> (-1.05%)	⬇️
pkg/btree/btree.go	85.30% <0.00%> (-0.97%)	⬇️
server/schedule/operator/step.go	67.77% <0.00%> (-0.67%)	⬇️
server/grpc_service.go	48.57% <0.00%> (-0.23%)	⬇️
server/tso/allocator_manager.go	79.13% <0.00%> (+0.16%)	⬆️
server/server.go	72.07% <0.00%> (+0.75%)	⬆️
server/tso/tso.go	74.15% <0.00%> (+4.49%)	⬆️
... and 3 more

---

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: f387675