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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