Add metadata to snapshots.

src/Nest/Modules/SnapshotAndRestore/Snapshot/Snapshot.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.Serialization;
+using Elasticsearch.Net;
 
 namespace Nest
 {
@@ -35,5 +36,8 @@ public class Snapshot
 
 		[DataMember(Name ="state")]
 		public string State { get; internal set; }
+
+		[DataMember(Name ="metadata")]
+		public IReadOnlyDictionary<string, object> Metadata { get; internal set; } = EmptyReadOnly<string, object>.Dictionary;
 	}
 }

src/Nest/Modules/SnapshotAndRestore/Snapshot/Snapshot/SnapshotRequest.cs

@@ -1,4 +1,6 @@
-using System.Runtime.Serialization;
+using System;
+using System.Collections.Generic;
+using System.Runtime.Serialization;
 using Elasticsearch.Net.Utf8Json;
 
 namespace Nest
@@ -18,6 +20,9 @@ public partial interface ISnapshotRequest
 
 		[DataMember(Name ="partial")]
 		bool? Partial { get; set; }
+
+		[DataMember(Name = "metadata")]
+		IDictionary<string, object> Metadata { get; set; }
 	}
 
 	public partial class SnapshotRequest
@@ -28,6 +33,8 @@ public partial class SnapshotRequest
 		public Indices Indices { get; set; }
 
 		public bool? Partial { get; set; }
+
+		public IDictionary<string, object> Metadata { get; set; }
 	}
 
 	public partial class SnapshotDescriptor
@@ -39,6 +46,8 @@ public partial class SnapshotDescriptor
 
 		bool? ISnapshotRequest.Partial { get; set; }
 
+		IDictionary<string, object> ISnapshotRequest.Metadata { get; set; }
+
 		public SnapshotDescriptor Index(IndexName index) => Indices(index);
 
 		public SnapshotDescriptor Index<T>() where T : class => Indices(typeof(T));
@@ -50,5 +59,10 @@ public partial class SnapshotDescriptor
 		public SnapshotDescriptor IncludeGlobalState(bool? includeGlobalState = true) => Assign(includeGlobalState, (a, v) => a.IncludeGlobalState = v);
 
 		public SnapshotDescriptor Partial(bool? partial = true) => Assign(partial, (a, v) => a.Partial = v);
+
+		public SnapshotDescriptor Metadata(IDictionary<string, object> metadata) => Assign(metadata, (a, v) => a.Metadata = v);
+
+		public SnapshotDescriptor Metadata(Func<FluentDictionary<string, object>, IDictionary<string, object>> selector) =>
+			Assign(selector, (a, v) => a.Metadata = v?.Invoke(new FluentDictionary<string, object>()));
 	}
 }