Clarify that heap usage is a minimum

Author: nicktindall

server/src/main/java/org/elasticsearch/cluster/ClusterInfo.java

@@ -72,6 +72,7 @@ protected ClusterInfo() {
      * @param shardDataSetSizes a shard id to data set size in bytes mapping per shard
      * @param dataPath the shard routing to datapath mapping
      * @param reservedSpace reserved space per shard broken down by node and data path
+     * @param nodesHeapUsage heap usage broken down by node
      * @see #shardIdentifierFromRouting
      */
     public ClusterInfo(
@@ -216,6 +217,15 @@ public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params params
         );
     }
 
+    /**
+     * Returns a node id to estimated heap usage mapping for all nodes that we have such data for.
+     * Note that these estimates should be considered minimums. They may be used to determine whether
+     * there IS NOT capacity to do something, but not to determine that there IS capacity to do something.
+     */
+    public Map<String, HeapUsage> getNodesHeapUsage() {
+        return nodesHeapUsage;
+    }
+
     /**
      * Returns a node id to disk usage mapping for the path that has the least available space on the node.
      * Note that this does not take account of reserved space: there may be another path with less available _and unreserved_ space.