Add ml.get_model_snapshot_upgrade_stats (#1732)

* Add request & response for ml.get_model_snapshot_upgrade_stats

* Make contrib

* Upgrade ModelSnapshotUpgrade.status to enum

* Fix typo

Author: Anaethelion

output/schema/schema.json

@@ -20,6 +20,7 @@
 import { ByteSize, Id, VersionString } from '@_types/common'
 import { integer, long } from '@_types/Numeric'
 import { Time } from '@_types/Time'
+import { DiscoveryNode } from '@ml/_types/DiscoveryNode'
 
 export class ModelSnapshot {
   /** An optional description of the job. */
@@ -44,6 +45,14 @@ export class ModelSnapshot {
   timestamp: long
 }
 
+export class ModelSnapshotUpgrade {
+  job_id: Id
+  snapshot_id: Id
+  state: SnapshotUpgradeState
+  node: DiscoveryNode
+  assignment_explanation: string
+}
+
 export class ModelSizeStats {
   bucket_allocation_failures_count: long
   job_id: Id
@@ -78,3 +87,10 @@ export enum MemoryStatus {
   soft_limit,
   hard_limit
 }
+
+export enum SnapshotUpgradeState {
+  loading_old_state,
+  saving_new_state,
+  stopped,
+  failed
+}