Add shard explain info to ReactiveReason about unassigned shards

[arteam]

Add two new fields into the reactive autoscaling policy decision response.

• unassigned_node_decisions - can_allocate decisions for the 1st unassigned shard
• assigned_node_decisions - can_remain decisions for the 1st assigned shard

Resolves #85243

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[elasticsearchmachine]

Hi @arteam, I've created a changelog YAML for you.

[arteam]

@elasticsearchmachine run elasticsearch-ci/docs-check

[arteam]

Hey @fcofdez and @henningandersen! I've addressed the comments from the last batch,

I created a new class NodeDecision to represent an abstraction holding a node and a decision instead of reusing NodeAllocationResult. NodeAllocationResult is suppose to work only with can_allocate decisions and we needed the class to hold also can_remain decisions.

I've also added suggestions from Henning for adding can_allocate decision for the nodes on the tier, if a shard can't be moved, and also I added a can_remain decision for the node if it can't be allocated due to storage.

[arteam]

@elasticmachine run elasticsearch-ci/part-2

[arteam]

@elasticmachine run elasticsearch-ci/bwc

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[arteam]

@elasticmachine update branch

[elasticmachine]

merge conflict between base and head

[arteam]

@elasticmachine update branch

[arteam]

Thank you both!