Add per-field metadata (#4376)

Relates: #4341

This commit adds the ability to define per-field metadata in mappings,
which will be returned via the Get Mapping API and Field Capabilities API.

src/Nest/Mapping/AttributeBased/ElasticsearchPropertyAttributeBase.cs

@@ -23,6 +23,8 @@ public abstract class ElasticsearchPropertyAttributeBase : Attribute, IProperty,
 
 		IDictionary<string, object> IProperty.LocalMetadata { get; set; }
 
+		IDictionary<string, string> IProperty.Meta { get; set; }
+
 		PropertyName IProperty.Name { get; set; }
 		private IProperty Self => this;
 		string IProperty.Type { get; set; }

src/Nest/Mapping/Types/PropertyBase.cs

@@ -20,6 +20,18 @@ public interface IProperty : IFieldMapping
 		[IgnoreDataMember]
 		IDictionary<string, object> LocalMetadata { get; set; }
 
+		/// <summary>
+		/// Metadata attached to the field. This metadata is stored in but opaque to Elasticsearch. It is
+		/// only useful for multiple applications that work on the same indices to share
+		/// meta information about fields such as units.
+		///<para></para>
+		/// Field metadata enforces at most 5 entries, that keys have a length that
+		/// is less than or equal to 20, and that values are strings whose length is less
+		///	than or equal to 50.
+		/// </summary>
+		[DataMember(Name = "meta")]
+		IDictionary<string, string> Meta { get; set; }
+
 		/// <summary>
 		/// The name of the property
 		/// </summary>
@@ -53,11 +65,14 @@ public abstract class PropertyBase : IProperty, IPropertyWithClrOrigin
 		/// <inheritdoc />
 		public IDictionary<string, object> LocalMetadata { get; set; }
 
+		/// <inheritdoc />
+		public IDictionary<string, string> Meta { get; set; }
+
 		/// <inheritdoc />
 		public PropertyName Name { get; set; }
 
 		protected string DebugDisplay => $"Type: {((IProperty)this).Type ?? "<empty>"}, Name: {Name.DebugDisplay} ";
-		
+
 		public override string ToString() => DebugDisplay;
 
 		/// <summary>

src/Nest/Mapping/Types/PropertyDescriptorBase.cs

@@ -25,6 +25,7 @@ protected string TypeOverride
 
 		IDictionary<string, object> IProperty.LocalMetadata { get; set; }
 		PropertyName IProperty.Name { get; set; }
+		IDictionary<string, string> IProperty.Meta { get; set; }
 
 		string IProperty.Type
 		{
@@ -41,5 +42,9 @@ string IProperty.Type
 		/// <inheritdoc cref="IProperty.LocalMetadata" />
 		public TDescriptor LocalMetadata(Func<FluentDictionary<string, object>, FluentDictionary<string, object>> selector) =>
 			Assign(selector, (a, v) => a.LocalMetadata = v?.Invoke(new FluentDictionary<string, object>()));
+
+		/// <inheritdoc cref="IProperty.Meta" />
+		public TDescriptor Meta(Func<FluentDictionary<string, string>, FluentDictionary<string, string>> selector) =>
+			Assign(selector, (a, v) => a.Meta = v?.Invoke(new FluentDictionary<string, string>()));
 	}
 }

src/Nest/Mapping/Types/PropertyFormatter.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Elasticsearch.Net.Utf8Json;
 using Elasticsearch.Net.Utf8Json.Internal;
 using Elasticsearch.Net.Utf8Json.Resolvers;

src/Nest/Search/FieldCapabilities/FieldCapabilitiesResponse.cs

@@ -88,5 +88,8 @@ public class FieldCapabilities
 
 		[DataMember(Name = "searchable")]
 		public bool Searchable { get; internal set; }
+
+		[DataMember(Name = "meta")]
+		public IReadOnlyDictionary<string, string[]> Meta { get; internal set; } = EmptyReadOnly<string, string[]>.Dictionary;
 	}
 }

tests/Tests/ClientConcepts/HighLevel/Serialization/ExtendingNestTypes.doc.cs

@@ -35,6 +35,7 @@ public class ExtendingNestTypes
 		public class MyPluginProperty : IProperty
 		{
 			IDictionary<string, object> IProperty.LocalMetadata { get; set; }
+			IDictionary<string, string> IProperty.Meta { get; set; }
 			public string Type { get; set; } = "my_plugin_property";
 			public PropertyName Name { get; set; }
 

tests/Tests/Mapping/LocalMetadata/LocalMetadataVisitorTests.cs

@@ -4,7 +4,8 @@
 using Nest;
 using Tests.Mapping.Types.Core.Text;
 
-namespace Tests.Mapping.LocalMetadata {
+namespace Tests.Mapping.LocalMetadata
+{
 	public class LocalMetadataVisitorTests
 	{
 		[U]
@@ -17,7 +18,7 @@ public void CanAssignAndAccessLocalMetadataInitializer()
 					.AddTestLocalMetadata()
 				)) as ITypeMapping;
 
-			var visitor = new LocalMatadataVisitor();
+			var visitor = new LocalMetadataVisitor();
 			var walker = new MappingWalker(visitor);
 			walker.Accept(descriptor.Properties);
 
@@ -36,7 +37,7 @@ public void CanAssignAndAccessLocalMetadataFluent()
 					)
 				)) as ITypeMapping;
 
-			var visitor = new LocalMatadataVisitor();
+			var visitor = new LocalMetadataVisitor();
 			var walker = new MappingWalker(visitor);
 			walker.Accept(descriptor.Properties);
 
@@ -63,7 +64,7 @@ public static TDescriptor AddTestLocalMetadata<TDescriptor>(this TDescriptor des
 		}
 	}
 
-	public class LocalMatadataVisitor : NoopMappingVisitor
+	public class LocalMetadataVisitor : NoopMappingVisitor
 	{
 		public int MetadataCount { get; set; }
 
@@ -77,4 +78,4 @@ public override void Visit(ITextProperty property)
 			property.LocalMetadata.Should().Contain("Test", "TestValue");
 		}
 	}
-}
+}

tests/Tests/Mapping/Meta/MetaMappingApiTests.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using Elastic.Xunit.XunitPlumbing;
+using FluentAssertions;
+using Nest;
+using Tests.ClientConcepts.HighLevel.CovariantHits;
+using Tests.Core.Extensions;
+using Tests.Core.ManagedElasticsearch.Clusters;
+using Tests.Domain;
+using Tests.Framework.EndpointTests.TestState;
+using Tests.Mapping.Types;
+
+namespace Tests.Mapping.Meta
+{
+	[SkipVersion("<7.6.0", "Meta added in Elasticsearch 7.6.0")]
+	public class MetaMappingApiTests
+		: PropertyTestsBase
+	{
+		public MetaMappingApiTests(WritableCluster cluster, EndpointUsage usage) : base(cluster, usage) { }
+
+		protected override Func<PropertiesDescriptor<Project>, IPromise<IProperties>> FluentProperties => p => p
+			.Number(n => n
+				.Name(nn => nn.Rank)
+				.Type(NumberType.Integer)
+				.Meta(m => m
+					.Add("unit", "popularity")
+				)
+			);
+		protected override IProperties InitializerProperties => new Properties<Project>
+		{
+			{ n => n.Rank, new NumberProperty(NumberType.Integer)
+				{
+					Meta = new Dictionary<string, string>
+					{
+						{ "unit", "popularity" }
+					}
+				}
+			}
+		};
+
+		protected override void ExpectResponse(PutMappingResponse response)
+		{
+			base.ExpectResponse(response);
+
+			// check the meta shows up in get mapping API
+			var getMappingResponse = Client.Indices.GetMapping<Project>(m => m.Index(CallIsolatedValue));
+			getMappingResponse.IsValid.Should().BeTrue();
+			var mappingMeta = getMappingResponse.Indices[CallIsolatedValue].Mappings.Properties["rank"].Meta;
+			mappingMeta.Should().NotBeNull().And.ContainKey("unit");
+			mappingMeta["unit"].Should().Be("popularity");
+
+			// check the meta shows up in field capabilities API
+			var fieldCapsResponse = Client.FieldCapabilities(CallIsolatedValue, f => f
+				.Fields<Project>(ff => ff.Rank)
+			);
+			fieldCapsResponse.IsValid.Should().BeTrue();
+			var meta = fieldCapsResponse.Fields["rank"].Integer.Meta;
+			meta.Should().NotBeNull().And.ContainKey("unit");
+			meta["unit"].Should().BeEquivalentTo("popularity");
+		}
+	}
+}