OnlineDDL: Add cut-over-threshold flag to ApplySchema client command

[mattlord]

Description

There are two different ways to execute OnlineDDL or "online schema migrations" in Vitess:

• A client command using vtctlidclient with the ApplySchema

  vtctldclient ApplySchema --ddl-strategy "vitess"  --sql "ALTER TABLE test1 ADD age int" commerce
  

• SQL via vtgate

  mysql commerce -e "set @@ddl_strategy='vitess'; ALTER TABLE test1 ADD age int;"
  

Those two methods produce the exact same results within Vitess. There are a number of additional flags that you can specify as part of the ddl_strategy however: https://vitess.io/docs/user-guides/schema-changes/ddl-strategy-flags/

A key flag that you can specify as part of the ddl_strategy value is --cut-over-threshold, which limits the time we can spend cutting over a single migration before giving up and retrying again later. The primary thing that this limits within that goal is how long we wait for the replication being used for the migration (from the old table to the new) to catch up. If you are experiencing replication lag for any number of reasons, you may need to increase this value. You can do so via the SQL method today this way:

mysql commerce -e "set @@ddl_strategy='vitess --cut-over-threshold=30s'; ALTER TABLE test1 ADD age int;"

There is, however, no way to affect that with the vtctldclient method today. That's what this PR adds.

Related Issue(s)

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[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.15%. Comparing base (be0bca3) to head (fecc1f0).

Files with missing lines	Patch %	Lines
go/cmd/vtctldclient/command/schema.go	25.00%	3 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	33.33%	2 Missing ⚠️
go/vt/vttablet/onlineddl/executor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17061      +/-   ##
==========================================
- Coverage   67.16%   67.15%   -0.02%     
==========================================
  Files        1571     1571              
  Lines      252093   252098       +5     
==========================================
- Hits       169314   169289      -25     
- Misses      82779    82809      +30     

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[mattlord]

I'm closing this as what one should do instead is pass this ddl-strategy option and any others as part of the --ddl-strategy flag. For example:

vtctldclient ApplySchema --ddl-strategy "vitess --cut-over-threshold=5m" --sql "ALTER TABLE test1 ADD age int" commerce