[Feature]: Respect `nodes.target` during fleet in-place update

[jvstme]

Problem

nodes.target is ignored when performing fleet in-place update. Respecting nodes.target will allow to:

• Pre-provision more instances into an existing fleet.
• Terminate excessive idle fleet instances.

Solution

Respect nodes.target during fleet in-place update.

• If nodes.target is greater than the current number of instances, provision new ones to reach nodes.target.
• If nodes.target is less than the current number of instances, terminate some of the idle ones to get as close as possible to nodes.target. Do not terminate busy instances.
• If nodes.target is equal to the current number of instances or unspecified, do nothing.

Workaround

• For pre-provisioning ­— increase nodes.min, apply, revert nodes.min, apply.
• For terminating — dstack fleet delete -i ...

Would you like to help us implement this feature by sending a PR?

Yes

[jvstme]

@r4victor, does this match what you have in mind for fleet in-place update?

[r4victor]

Yes, I was thinking about it when doing #3775 but decided to do it separately since " terminate some of the idle ones" will require locking instances in fleet apply, which is not currently necessary for other in-place updates.