Improve VTOrc config handling to support dynamic variables

[GuptaManan100]

Description

This PR changes how config files are handled in VTOrc, moving them to use viper instead of the custom solution previously employed. This change allows the following parameters to be dynamically loaded -

1. instance-poll-time
2. prevent-cross-cell-failover
3. snapshot-topology-interval
4. reasonable-replication-lag
5. audit-to-backend
6. audit-to-syslog
7. audit-purge-duration
8. wait-replicas-timeout
9. tolerable-replication-lag
10. topo-information-refresh-duration
11. recovery-poll-duration
12. allow-emergency-reparent
13. change-tablets-with-errant-gtid-to-drained

Related Issue(s)

• Fixes Feature Request: Handle config in VTOrc using Viper #17217

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 41.48148% with 79 lines in your changes missing coverage. Please review.

Project coverage is 67.39%. Comparing base (0726ea6) to head (85fbc91).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/config/config.go	36.58%	52 Missing ⚠️
go/vt/vtorc/server/api.go	9.09%	10 Missing ⚠️
go/vt/vtorc/logic/vtorc.go	0.00%	6 Missing ⚠️
go/viperutil/debug/debug.go	0.00%	4 Missing ⚠️
go/cmd/vtorc/cli/cli.go	0.00%	2 Missing ⚠️
go/vt/servenv/servenv.go	81.81%	2 Missing ⚠️
go/vt/vtorc/discovery/queue.go	0.00%	1 Missing ⚠️
go/vt/vtorc/inst/instance_dao.go	87.50%	1 Missing ⚠️
go/vt/vtorc/logic/tablet_discovery.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17218      +/-   ##
==========================================
- Coverage   67.40%   67.39%   -0.02%     
==========================================
  Files        1574     1574              
  Lines      253221   253251      +30     
==========================================
- Hits       170690   170670      -20     
- Misses      82531    82581      +50     

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[GuptaManan100]

This makes the field dynamic

[timvaillancourt]

@GuptaManan100 double-checking: the 3rd party library handles SIGHUP reload?

[GuptaManan100]

SIGHUP is not even required anymore. Viper has a watcher on the file, and if it changes, it automatically reloads the configurations.

[timvaillancourt]

@GuptaManan100 looks good to me, added one question re: SIGHUP reload that I assume still functions

[deepthi]

We should add tests to replace the ones that were deleted. Ideally an e2e test that changes the file contents and verifies that the in-memory value does change.

[GuptaManan100]

I added a e2e test to check that the configs are indeed reloading dynamically. I, however, only added it for one of the configs. Would you prefer if I added one for each of the reloadable configurations?

[deepthi]

I added a e2e test to check that the configs are indeed reloading dynamically. I, however, only added it for one of the configs. Would you prefer if I added one for each of the reloadable configurations?

we can add all of the configs into the same test.

[deepthi]

A couple of small suggestions. Otherwise LGTM