Remove uses of ImmutableOpen*Map

[rjernst]

After https://github.com/elastic/elasticsearch/issues/84735, hppc usage in Elasticsearch is now isolated to ImmutableOpenMap and ImmutableOpenIntMap. This is a meta issue delineating the remaining uses of those classes that need to be removed.

• [ ] ImmutableOpenIntMap (https://github.com/elastic/elasticsearch/pull/86447)
  • [x] server
  • [x] o.e.Version (https://github.com/elastic/elasticsearch/pull/86245)
  • [x] o.e.action.admin.indices.shards (https://github.com/elastic/elasticsearch/pull/86281)
  • [x] o.e.cluster.metadata.IndexMetadata (https://github.com/elastic/elasticsearch/pull/86403)
  • [x] o.e.cluster.routing.IndexRoutingTable (https://github.com/elastic/elasticsearch/pull/86368)
  • [x] server tests
  • [x] o.e.action.admin.indices.shards (https://github.com/elastic/elasticsearch/pull/86281)
  • [x] o.e.cluster.health (https://github.com/elastic/elasticsearch/pull/86306)
  • [x] server integ tests
  • [x] o.e.action.admin.indices.shards (https://github.com/elastic/elasticsearch/pull/86281)
  • [x] o.e.cluster.routing (https://github.com/elastic/elasticsearch/pull/86368)
• [ ] ImmutableOpenMap exposed by public methods
  • [x] o.e.action.admin.indices (https://github.com/elastic/elasticsearch/pull/87292)
  • [x] o.e.cluster.ClusterState (https://github.com/elastic/elasticsearch/pull/87265)
  • [ ] other o.e.cluster (https://github.com/elastic/elasticsearch/pull/87168)
• [ ] general ImmutableOpenMap usages
  • [x] server tests (https://github.com/elastic/elasticsearch/pull/89864, https://github.com/elastic/elasticsearch/pull/89939)
  • [ ] server integ tests
  • [ ] test framework
  • [ ] xpack core
  • [ ] data streams
  • [ ] autoscaling
  • [ ] ccr
  • [ ] deprecation
  • [ ] enrich
  • [ ] ilm tests
  • [ ] ML
  • [ ] QL
  • [ ] rollup
  • [ ] searchable snapshots
  • [ ] security
  • [ ] node shutdown
  • [ ] transforms
  • [ ] watcher

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[dakrone]

@rjernst do we still plan to keep some (most?) of these as immutable maps, or are we converting to mutable HashMap-like maps for these?

[rjernst]

Yes I think they can and should remain immutable, just immutable Map instances with Map.of or Map.copyOf.

[thecoop]

ImmutableOpenMap was added to the cluster/index metadata classes in #86673 for performance reasons. So we need to find a different collection type (maybe guava immutablemap?) if we want to remove ImmutableOpenMap

[thecoop]

https://github.com/elastic/elasticsearch/pull/90006 caused a performance drop, needs to be thought about more