Add reserved, reserved_in_bytes to node stats response (#4918)

Relates: elastic/elasticsearch#58029

Author: russcam

src/Nest/CommonOptions/Stats/StoreStats.cs

@@ -9,10 +9,35 @@ namespace Nest
 	[DataContract]
 	public class StoreStats
 	{
-		[DataMember(Name ="size")]
+		/// <summary>
+		/// Total size of all shards assigned to the node.
+		/// </summary>
+		[DataMember(Name = "size")]
 		public string Size { get; set; }
 
-		[DataMember(Name ="size_in_bytes")]
+		/// <summary>
+		/// Total size, in bytes, of all shards assigned to the node.
+		/// </summary>
+		// TODO: should be long
+		[DataMember(Name = "size_in_bytes")]
 		public double SizeInBytes { get; set; }
+
+		/// <summary>
+		/// A prediction of how much larger the shard stores on this node will eventually grow due to ongoing peer recoveries, restoring snapshots,
+		/// and similar activities. A value of -1b indicates that this is not available.
+		/// <para />
+		/// Valid in Elasticsearch 7.9.0+
+		/// </summary>
+		[DataMember(Name = "reserved")]
+		public string Reserved { get; set; }
+
+		/// <summary>
+		/// A prediction, in bytes, of how much larger the shard stores on this node will eventually grow due to ongoing peer recoveries,
+		/// restoring snapshots, and similar activities. A value of -1 indicates that this is not available.
+		/// <para />
+		/// Valid in Elasticsearch 7.9.0+
+		/// </summary>
+		[DataMember(Name = "reserved_in_bytes")]
+		public long ReservedInBytes { get; set; }
 	}
 }

tests/Tests/Cluster/NodesStats/NodesStatsApiTests.cs

@@ -122,7 +122,10 @@ protected void Assert(IndexStats index)
 			//index.Documents.Count.Should().BeGreaterThan(0);
 
 			index.Store.Should().NotBeNull();
-			//index.Store.SizeInBytes.Should().BeGreaterThan(0);
+			index.Store.SizeInBytes.Should().BeGreaterOrEqualTo(0);
+
+			if (TestClient.Configuration.InRange(">=7.9.0"))
+				index.Store.ReservedInBytes.Should().BeGreaterOrEqualTo(0);
 
 			index.Completion.Should().NotBeNull();
 			index.Fielddata.Should().NotBeNull();