Add VStreamerCount metric from vitessio/vitess pr #11978

[timvaillancourt]

This PR adds a description to metrics.md in v16 for the new VStreamerCount vreplication metric

This is planned to be added in PR vitessio/vitess#11978

cc @mattlord 🙇

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[mattlord]

LGTM! https://deploy-preview-1291--vitess.netlify.app/docs/16.0/reference/vreplication/metrics/#vstreamercount

I want to get people more involved with the docs, I love this. It's a relatively easy way to contribute while also having such a major positive impact on the project and its users. 🙂 ❤️

Note we'll need to address the DCO failure before we can merge: https://github.com/vitessio/website/pull/1291/checks?check_run_id=10146299424

[timvaillancourt]

I want to get people more involved with the docs, I love this. It's a relatively easy way to contribute while also having such a major positive impact on the project and its users. 🙂 ❤️

Agreed! We ask contributors to add to markdown docs in github/gh-ost and it has been a pretty smooth process like this one 👍