[Backport master] Fix failing transform tests and support dates_as_epoch_millis (#5212)

Tests were failing due to a change in the transform API which now
prefers ISO dates, rather than epoch milliseconds for dates. This
resulted in a serialisation failure.

This commit addresses that by conditionally producing the requests
based on the version of ES being tested, so that a property of the
appropriate type is used for deserialisation of the preview.

This also adds support for the now dates_as_epoch_millis setting which
allows the original epoch millisecond format to be returned for ES 7.11
requests, when the option is true.

(cherry picked from commit 7994d6f)

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

Author: Mpdreamz

src/Nest/XPack/Transform/TransformSettings.cs

@@ -21,6 +21,9 @@ public interface ITransformSettings
 
 		[DataMember(Name = "max_page_search_size")]
 		public int? MaxPageSearchSize { get; set; }
+
+		[DataMember(Name = "dates_as_epoch_millis")]
+		public bool? DatesAsEpochMilliseconds { get; set; }
 	}
 
 	/// <inheritdoc />
@@ -31,12 +34,16 @@ public class TransformSettings : ITransformSettings
 
 		/// <inheritdoc />
 		public int? MaxPageSearchSize { get; set; }
+
+		/// <inheritdoc />
+		public bool? DatesAsEpochMilliseconds { get; set; }
 	}
 
 	public class TransformSettingsDescriptor : DescriptorBase<TransformSettingsDescriptor, ITransformSettings>, ITransformSettings
 	{
 		float? ITransformSettings.DocsPerSecond { get; set; }
 		int? ITransformSettings.MaxPageSearchSize { get; set; }
+		bool? ITransformSettings.DatesAsEpochMilliseconds { get; set; }
 
 		/// <inheritdoc cref="ITransformSettings.DocsPerSecond"/>
 		public TransformSettingsDescriptor DocsPerSecond(float? docsPerSecond) =>
@@ -45,5 +52,9 @@ public TransformSettingsDescriptor DocsPerSecond(float? docsPerSecond) =>
 		/// <inheritdoc cref="ITransformSettings.MaxPageSearchSize"/>
 		public TransformSettingsDescriptor MaxPageSearchSize(int? maxPageSearchSize) =>
 			Assign(maxPageSearchSize, (a, v) => a.MaxPageSearchSize = v);
+
+		/// <inheritdoc cref="ITransformSettings.DatesAsEpochMilliseconds"/>
+		public TransformSettingsDescriptor DatesAsEpochMilliseconds(bool? datesAsEpochMillis = true) =>
+			Assign(datesAsEpochMillis, (a, v) => a.DatesAsEpochMilliseconds = v);
 	}
 }

tests/Tests.Domain/Project.cs

@@ -196,8 +196,10 @@ public class Metadata
 	public class ProjectTransform
 	{
 		public double? AverageCommits { get; set; }
+		
+		public long WeekStartedOnMillis { get; set; }
 
-		public long WeekStartedOn { get; set; }
+		public DateTime WeekStartedOnDate { get; set; }
 
 		public long SumIntoMaster { get; set; }
 	}

tests/Tests/XPack/Transform/TransformApiTests.cs

@@ -1,9 +1,11 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Elastic.Elasticsearch.Xunit.XunitPlumbing;
 using FluentAssertions;
 using Nest;
+using Tests.Core.Client;
 using Tests.Core.Extensions;
 using Tests.Core.ManagedElasticsearch.Clusters;
 using Tests.Domain;
@@ -171,7 +173,7 @@ public TransformApiTests(WritableCluster cluster, EndpointUsage usage) : base(ne
 									},
 								GroupBy = new Dictionary<string, ISingleGroupSource>
 								{
-									{ "weekStartedOn", new DateHistogramGroupSource()
+									{ TestClient.Configuration.InRange("<7.11.0") ? "weekStartedOnMillis" : "weekStartedOnDate", new DateHistogramGroupSource
 										{
 											Field = Field<Project>(f => f.StartedOn),
 											CalendarInterval = DateInterval.Week
@@ -207,7 +209,7 @@ public TransformApiTests(WritableCluster cluster, EndpointUsage usage) : base(ne
 									)
 								)
 								.GroupBy(g => g
-									.DateHistogram("weekStartedOn", dh => dh
+									.DateHistogram(TestClient.Configuration.InRange("<7.11.0") ? "weekStartedOnMillis" : "weekStartedOnDate", dh => dh
 										.Field(f => f.StartedOn)
 										.CalendarInterval(DateInterval.Week)
 									)
@@ -317,6 +319,11 @@ [I] public async Task PreviewTransformResponse() => await Assert<PreviewTransfor
 			r.GeneratedDestinationIndex.Mappings.Should().NotBeNull();
 			r.GeneratedDestinationIndex.Settings.Should().NotBeNull();
 			r.GeneratedDestinationIndex.Aliases.Should().NotBeNull();
+
+			if (TestClient.Configuration.InRange("<7.11.0"))
+				r.Preview.First().WeekStartedOnMillis.Should().BeGreaterOrEqualTo(1);
+			else
+				r.Preview.First().WeekStartedOnDate.Should().NotBe(DateTime.MinValue);
 		});
 
 		[I] public async Task UpdateTransformResponse() => await Assert<UpdateTransformResponse>(UpdateTransformStep, (v, r) =>