Add formatter for int -> string

src/Nest/CommonAbstractions/SerializationBehavior/JsonFormatters/IntStringFormatter.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Globalization;
+using Elasticsearch.Net.Utf8Json;
+
+namespace Nest
+{
+	/// <summary>
+	/// A formatter to deserialize an int into a string,
+	/// and serialize a string into an int.
+	/// </summary>
+	internal class IntStringFormatter: IJsonFormatter<string>
+	{
+		public void Serialize(ref JsonWriter writer, string value, IJsonFormatterResolver formatterResolver)
+		{
+			if (int.TryParse(value, out var i))
+				writer.WriteInt32(i);
+			else
+				throw new InvalidOperationException($"expected a int string value, but found {value}");
+		}
+
+		public string Deserialize(ref JsonReader reader, IJsonFormatterResolver formatterResolver)
+		{
+			switch (reader.GetCurrentJsonToken())
+			{
+				case JsonToken.Number:
+					return reader.ReadInt32().ToString(CultureInfo.InvariantCulture);
+				case JsonToken.String:
+					return reader.ReadString();
+				default:
+					throw new JsonParsingException($"expected string or int but found {reader.GetCurrentJsonToken()}");
+			}
+		}
+	}
+}

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

@@ -1,4 +1,5 @@
 using System.Runtime.Serialization;
+using Elasticsearch.Net.Utf8Json;
 
 namespace Nest
 {
@@ -15,6 +16,7 @@ public class SnapshotShardFailure
 		public string Reason { get; set; }
 
 		[DataMember(Name ="shard_id")]
+		[JsonFormatter(typeof(IntStringFormatter))]
 		public string ShardId { get; set; }
 
 		[DataMember(Name ="status")]

tests/Tests.Reproduce/GitHubIssue4537.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Linq;
+using System.Text;
+using Elastic.Xunit.XunitPlumbing;
+using FluentAssertions;
+using Nest;
+using Tests.Core.Client;
+
+namespace Tests.Reproduce
+{
+	public class GitHubIssue4537
+	{
+		[U]
+		public void CanDeserializeSnapshotShardFailure()
+		{
+			var json = @"{
+						  ""snapshots"": [
+						    {
+						      ""snapshot"": ""snapshot_2020-03-31t00:02:18z"",
+						      ""uuid"": ""P9oZzuEfS8qbT-FVZLFSgw"",
+						      ""version_id"": 7040299,
+						      ""version"": ""7.4.2"",
+						      ""indices"": [ ""someIndices"" ],
+						      ""include_global_state"": true,
+						      ""state"": ""FAILED"",
+						      ""reason"": ""Indices don't have primary shards [someIndex]"",
+						      ""start_time"": ""2020-03-31T00:02:21.478Z"",
+						      ""start_time_in_millis"": 1585612941478,
+						      ""end_time"": ""2020-03-31T00:02:25.353Z"",
+						      ""end_time_in_millis"": 1585612945353,
+						      ""duration_in_millis"": 3875,
+						      ""failures"": [
+						        {
+						          ""index"": ""someIndex"",
+						          ""index_uuid"": ""someIndex"",
+						          ""shard_id"": 1,
+						          ""reason"": ""primary shard is not allocated"",
+						          ""status"": ""INTERNAL_SERVER_ERROR""
+						        },
+						        {
+						          ""index"": ""someIndex"",
+						          ""index_uuid"": ""someIndex"",
+						          ""shard_id"": 0,
+						          ""reason"": ""primary shard is not allocated"",
+						          ""status"": ""INTERNAL_SERVER_ERROR""
+						        },
+						        {
+						          ""index"": ""someIndex"",
+						          ""index_uuid"": ""someIndex"",
+						          ""shard_id"": 2,
+						          ""reason"": ""primary shard is not allocated"",
+						          ""status"": ""INTERNAL_SERVER_ERROR""
+						        }
+						      ],
+						      ""shards"": {
+						        ""total"": 78,
+						        ""failed"": 3,
+						        ""successful"": 75
+						      }
+						    }
+						  ]
+						}";
+
+			var client = TestClient.FixedInMemoryClient(Encoding.UTF8.GetBytes(json));
+
+			Func<GetSnapshotResponse> action = () => client.Snapshot.Get("repo", "snapshot_2020-03-31t00:02:18z");
+
+			action.Should().NotThrow();
+
+			var response = action();
+
+			var failures = response.Snapshots.First().Failures;
+			failures.Should().HaveCount(3);
+			failures.First().ShardId.Should().Be("1");
+		}
+	}
+}