Migrates cmd/vtbench to pflags

[frouioui]

Description

This PR changes cmd/vtbench to use pflags as instructed in #10697.

Related Issue(s)

• Fixes Switch flag definitions to be on pflag instead of flag in package go/cmd/vtbench #11287

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should either include a link to an issue that describes the bug OR an actual description of the bug and how to reproduce, along with a description of the fix.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.

[frouioui]

@ajm188, I was not sure whether this needs to be removed or not. I can see that #11364 is getting rid of it. Let me know what I should follow :)

[ajm188]

well, it depends on what you want 😅

if you want to keep the behavior, you want to something like:

servenv.OnParse(func(fs *pflag.FlagSet) {
    fs.SetOutput(logutil.NewLoggerWriter(logger))
}

[GuptaManan100]

LGTM!