Add new CachePopulationReason

Author: fcofdez

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/BlobCacheMetrics.java

@@ -47,7 +47,11 @@ public enum CachePopulationReason {
         /**
          * When the data we need is not in the cache
          */
-        CacheMiss
+        CacheMiss,
+        /**
+         * When data is prefetched upon new commit notifications
+         */
+        PreFetching
     }
 
     public BlobCacheMetrics(MeterRegistry meterRegistry) {