Add OnlinePrewarming option for cache population reason (#126716)

Author: andreidan

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

@@ -40,6 +40,10 @@ public enum CachePopulationReason {
          * When warming the cache
          */
         Warming,
+        /**
+         * When warming the cache as a result of an incoming request
+         */
+        OnlinePrewarming,
         /**
          * When the data we need is not in the cache
          */