[slack-19.0]Backport grpc metrics for load balancing

[twthorn]

Description

Backport two PRs
vitessio#18282
vitessio#18791

These will allow for grpc metrics to be used by a client to load balance dynamically and lead to a more even distribution of vstreams to vtgates.

Related Issue(s)

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

AI Disclosure

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @mattlord to sign the Salesforce Inc. Contributor License Agreement.

[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Thomas Thornton <t***@s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, refresh the status of this Pull Request.