[Backport master] Add support for rate aggregation (#5242)

* Add support for rate aggregation (#5206)

* Add support for rate aggregation
* Add missing XML comments

* Fix namespace in master

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

Author: github-actions[bot]

docs/aggregations.asciidoc

@@ -56,6 +56,8 @@ The values are typically extracted from the fields of the document (using the fi
 
 * <<percentiles-aggregation-usage,Percentiles Aggregation Usage>>
 
+* <<rate-aggregation-usage,Rate Aggregation Usage>>
+
 * <<scripted-metric-aggregation-usage,Scripted Metric Aggregation Usage>>
 
 * <<stats-aggregation-usage,Stats Aggregation Usage>>
@@ -100,6 +102,8 @@ include::aggregations/metric/percentile-ranks/percentile-ranks-aggregation-usage
 
 include::aggregations/metric/percentiles/percentiles-aggregation-usage.asciidoc[]
 
+include::aggregations/metric/rate/rate-aggregation-usage.asciidoc[]
+
 include::aggregations/metric/scripted-metric/scripted-metric-aggregation-usage.asciidoc[]
 
 include::aggregations/metric/stats/stats-aggregation-usage.asciidoc[]

docs/aggregations/metric/rate/rate-aggregation-usage.asciidoc

@@ -0,0 +1,94 @@
+:ref_current: https://www.elastic.co/guide/en/elasticsearch/reference/7.11
+
+:github: https://github.com/elastic/elasticsearch-net
+
+:nuget: https://www.nuget.org/packages
+
+////
+IMPORTANT NOTE
+==============
+This file has been generated from https://github.com/elastic/elasticsearch-net/tree/7.x/src/Tests/Tests/Aggregations/Metric/Rate/RateAggregationUsageTests.cs. 
+If you wish to submit a PR for any spelling mistakes, typos or grammatical errors for this file,
+please modify the original csharp file found at the link and submit the PR with that change. Thanks!
+////
+
+[[rate-aggregation-usage]]
+=== Rate Aggregation Usage
+
+A rate metrics aggregation can be used only inside a date_histogram and calculates a rate of documents or a field in each
+date_histogram bucket. The field values can be generated by a provided script or extracted from specific numeric or histogram fields in the documents.
+
+Be sure to read the Elasticsearch documentation on {ref_current}/search-aggregations-metrics-rate-aggregation.html[Rate Aggregation].
+
+==== Fluent DSL example
+
+[source,csharp]
+----
+a => a
+.DateHistogram("by_date", d => d
+    .Field(f => f.StartedOn)
+    .CalendarInterval(DateInterval.Month)
+    .Aggregations(a => a
+        .Rate("my_rate", m => m
+            .Field(p => p.NumberOfCommits)
+            .Unit(DateInterval.Month)
+            .Mode(RateMode.Sum)
+        )))
+----
+
+==== Object Initializer syntax example
+
+[source,csharp]
+----
+new DateHistogramAggregation("by_date")
+{
+    Field = Field<Project>(p => p.StartedOn),
+    CalendarInterval = DateInterval.Month,
+    Aggregations = new RateAggregation("my_rate", Field<Project>(p => p.NumberOfCommits))
+    {
+        Unit = DateInterval.Month,
+        Mode = RateMode.Sum
+    }
+}
+----
+
+[source,javascript]
+.Example json output
+----
+{
+  "by_date": {
+    "date_histogram": {
+      "field": "startedOn",
+      "calendar_interval": "month"
+    },
+    "aggs": {
+      "my_rate": {
+        "rate": {
+          "field": "numberOfCommits",
+          "unit": "month",
+          "mode": "sum"
+        }
+      }
+    }
+  }
+}
+----
+
+==== Handling Responses
+
+[source,csharp]
+----
+response.ShouldBeValid();
+
+var dateHistogram = response.Aggregations.DateHistogram("by_date");
+dateHistogram.Should().NotBeNull();
+dateHistogram.Buckets.Should().NotBeNull();
+dateHistogram.Buckets.Count.Should().BeGreaterThan(10);
+foreach (var item in dateHistogram.Buckets)
+{
+    var rate = item.Rate("my_rate");
+    rate.Should().NotBeNull();
+    rate.Value.Should().BeGreaterOrEqualTo(1);
+}
+----
+

src/Nest/Aggregations/AggregateDictionary.cs

@@ -205,6 +205,8 @@ public MultiBucketAggregate<RareTermsBucket<TKey>> RareTerms<TKey>(string key)
 				};
 		}
 
+		public ValueAggregate Rate(string key) => TryGet<ValueAggregate>(key);
+
 		public MultiBucketAggregate<RareTermsBucket<string>> RareTerms(string key) => RareTerms<string>(key);
 
 		public MultiBucketAggregate<RangeBucket> Range(string key) => GetMultiBucketAggregate<RangeBucket>(key);

src/Nest/Aggregations/AggregationContainer.cs

@@ -232,6 +232,9 @@ public interface IAggregationContainer
 		[DataMember(Name = "rare_terms")]
 		IRareTermsAggregation RareTerms { get; set; }
 
+		[DataMember(Name = "rate")]
+		IRateAggregation Rate { get; set; }
+
 		[DataMember(Name = "reverse_nested")]
 		IReverseNestedAggregation ReverseNested { get; set; }
 
@@ -397,6 +400,8 @@ public class AggregationContainer : IAggregationContainer
 
 		public IRareTermsAggregation RareTerms { get; set; }
 
+		public IRateAggregation Rate { get; set; }
+
 		public IReverseNestedAggregation ReverseNested { get; set; }
 
 		public ISamplerAggregation Sampler { get; set; }
@@ -559,6 +564,8 @@ public class AggregationContainerDescriptor<T> : DescriptorBase<AggregationConta
 
 		IRareTermsAggregation IAggregationContainer.RareTerms { get; set; }
 
+		IRateAggregation IAggregationContainer.Rate { get; set; }
+
 		IReverseNestedAggregation IAggregationContainer.ReverseNested { get; set; }
 
 		ISamplerAggregation IAggregationContainer.Sampler { get; set; }
@@ -733,6 +740,10 @@ Func<RareTermsAggregationDescriptor<T>, IRareTermsAggregation> selector
 		) =>
 			_SetInnerAggregation(name, selector, (a, d) => a.RareTerms = d);
 
+		public AggregationContainerDescriptor<T> Rate(string name,
+			Func<RateAggregationDescriptor<T>, IRateAggregation> selector) =>
+			_SetInnerAggregation(name, selector, (a, d) => a.Rate = d);
+
 		public AggregationContainerDescriptor<T> Stats(string name,
 			Func<StatsAggregationDescriptor<T>, IStatsAggregation> selector
 		) =>

src/Nest/Aggregations/Metric/Rate/RateAggregation.cs

@@ -0,0 +1,78 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+using System.Runtime.Serialization;
+using Elasticsearch.Net;
+using Nest.Utf8Json;
+
+namespace Nest
+{
+	[InterfaceDataContract]
+	[ReadAs(typeof(RateAggregation))]
+	public interface IRateAggregation : IMetricAggregation
+	{
+		/// <summary>
+		/// The <see cref="DateInterval"/> to use as the rate unit.
+		/// </summary>
+		[DataMember(Name = "unit")]
+		DateInterval? Unit { get; set; }
+
+		/// <summary>
+		/// The mode to use in the rate calculation. By default, "sum" mode is used.
+		/// </summary>
+		/// <remarks>
+		/// The mode may be either "sum", where the rate calculates the sum of field values, or
+		/// "value_count", where the rate uses the number of values in the field.
+		/// </remarks>
+		[DataMember(Name = "mode")]
+		RateMode? Mode { get; set; }
+	}
+
+	public class RateAggregation : MetricAggregationBase, IRateAggregation
+	{
+		public RateAggregation(string name) : base(name, null) { }
+		public RateAggregation(string name, Field field) : base(name, field) { }
+
+		internal override void WrapInContainer(AggregationContainer c) => c.Rate = this;
+
+		/// <inheritdoc cref ="IRateAggregation.Unit"/>
+		public DateInterval? Unit { get; set; }
+
+		/// <inheritdoc cref ="IRateAggregation.Mode"/>
+		public RateMode? Mode { get; set; }
+	}
+
+	public class RateAggregationDescriptor<T>
+		: MetricAggregationDescriptorBase<RateAggregationDescriptor<T>, IRateAggregation, T>, IRateAggregation
+		where T : class
+	{
+		DateInterval? IRateAggregation.Unit { get; set; }
+
+		RateMode? IRateAggregation.Mode { get; set; }
+		
+		/// <inheritdoc cref ="IRateAggregation.Unit"/>
+		public RateAggregationDescriptor<T> Unit(DateInterval? dateInterval) =>
+			Assign(dateInterval, (a, v) => a.Unit = v);
+
+		/// <inheritdoc cref ="IRateAggregation.Mode"/>
+		public RateAggregationDescriptor<T> Mode(RateMode? mode) =>
+			Assign(mode, (a, v) => a.Mode = v);
+	}
+
+	[StringEnum]
+	public enum RateMode
+	{
+		/// <summary>
+		/// Rate calculates the sum of field values.
+		/// </summary>
+		[EnumMember(Value = "sum")]
+		Sum,
+
+		/// <summary>
+		/// Rate uses the number of values in the field.
+		/// </summary>
+		[EnumMember(Value = "value_count")]
+		ValueCount
+	}
+}

src/Nest/Aggregations/Visitor/AggregationVisitor.cs

@@ -82,6 +82,8 @@ public interface IAggregationVisitor
 
 		void Visit(IRareTermsAggregation aggregation);
 
+		void Visit(IRateAggregation aggregation);
+
 		void Visit(ITermsAggregation aggregation);
 
 		void Visit(ISignificantTermsAggregation aggregation);
@@ -241,6 +243,8 @@ public virtual void Visit(IRangeAggregation aggregation) { }
 
 		public virtual void Visit(IRareTermsAggregation aggregation) { }
 
+		public virtual void Visit(IRateAggregation aggregation) { }
+
 		public virtual void Visit(INestedAggregation aggregation) { }
 
 		public virtual void Visit(INormalizeAggregation aggregation) { }

src/Nest/Aggregations/Visitor/AggregationWalker.cs

@@ -144,6 +144,7 @@ public void Walk(IAggregationContainer aggregation, IAggregationVisitor visitor)
 				v.Visit(d);
 				Accept(v, d.Aggregations);
 			});
+			AcceptAggregation(aggregation.Rate, visitor, (v, d) => v.Visit(d));
 			AcceptAggregation(aggregation.ReverseNested, visitor, (v, d) =>
 			{
 				v.Visit(d);