txthrottler: record replication lag on PRIMARY only

[timvaillancourt]

Description

This PR hopes to address #16243 by setting up healthcheck streamers (for gathering replication lag for throttling) on the PRIMARY only

To mimic other parts of vttablet, .MakePrimary() and .MakeNonPrimary() methods were added

Related Issue(s)

Checklist

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Deployment Notes

[vitess-bot]

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[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.

[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.