VStream API: allow cells to be specified for picking source tablets to stream from

[rohit-nayak-ps]

Description

The VTGate VStreamAPI, internally needs to source data from tablets using the RowStreamer and VStreamer APIs. It uses a TabletPicker to choose a tablet per shard. If a REPLICA tablet type is specified, currently the picker looks at tablets in the same cell as the vtgate to choose a replica to source from. However it is possible that there is no replica in the cell of the vtgate one connects to.

This PR adds the ability to specify a list of (comma separated) cell names/cell aliases. Instead of changing the API signature we add a new Cells field to the VStreamFlags struct for this purpose.

Sample usage:

options := &vtgatepb.VStreamFlags{
	Cells: "zone1, zone4"
}
reader, err := conn.VStream(ctx, topodatapb.TabletType_REPLICA, vgtid, filter, options)

The functionality is validated by a e2e test.

Related Issue(s)

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

[deepthi]

I assume this will also work with cell-aliases?

[rohit-nayak-ps]

I assume this will also work with cell-aliases?

Yes, because it is just calls the tablet picker with the cell names and we already resolve aliases there.

[github-actions]

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[rohit-nayak-ps]

@mattlord, @deepthi ping for review, thx.

[mattlord]

Sorry, @rohit-nayak-ps ! Somehow I completely missed this one. I will review in the next few hours.

[mattlord]

I had some questions and nits, but nothing major. Will approve so that you're not blocked if I misunderstood something or you disagree.