Add min and max threads parameters to tiered cache (#10276)

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

Author: kolchfa-aws

_search-plugins/caching/tiered-cache.md

@@ -67,6 +67,8 @@ Setting | Data type | Default | Description
 `indices.requests.cache.ehcache_disk.alias` | String | `ehcacheDiskCache#INDICES_REQUEST_CACHE` | Specifies an alias for the disk cache. Optional.
 `indices.requests.cache.ehcache_disk.segments` | Integer | `16` | Defines the number of segments into which the disk cache is separated. Used for concurrency. Optional.
 `indices.requests.cache.ehcache_disk.concurrency` | Integer | `1` | Defines the number of distinct write queues created for the disk store, where a group of segments shares a write queue. Optional.
+`indices.requests.cache.ehcache_disk.min_threads` | Integer | `2`  | Defines the minimum number of ehcache disk threads for the pool. Optional.
+`indices.requests.cache.ehcache_disk.max_threads` | Integer | The number of CPU cores  | Defines the maximum number of ehcache disk threads for the pool. The maximum allowed value is `10 * num_cpu_cores`. Disk operations are typically I/O bound rather than CPU bound, so you can set this value to a number greater than the number of CPU cores. Optional.
 
 ### Additional settings for the `tiered_spillover` store