Add PreserveOriginal to NGramTokenFilter (#4736)

Relates: #4718, elastic/elasticsearch#55432
Added in Elasticsearch 7.8.0

Author: russcam

src/Nest/Analysis/TokenFilters/NgramTokenFilter.cs

@@ -25,6 +25,15 @@ public interface INGramTokenFilter : ITokenFilter
 		[DataMember(Name ="min_gram")]
 		[JsonFormatter(typeof(NullableStringIntFormatter))]
 		int? MinGram { get; set; }
+
+		/// <summary>
+		/// Emits original token when set to `true`. Defaults to `false`.
+		/// <para />
+		/// Available in Elasticsearch 7.8.0+
+		/// </summary>
+		[DataMember(Name = "preserve_original")]
+		[JsonFormatter(typeof(NullableStringBooleanFormatter))]
+		bool? PreserveOriginal { get; set; }
 	}
 
 	/// <inheritdoc />
@@ -37,6 +46,9 @@ public NGramTokenFilter() : base("ngram") { }
 
 		/// <inheritdoc />
 		public int? MinGram { get; set; }
+
+		/// <inheritdoc />
+		public bool? PreserveOriginal { get; set; }
 	}
 
 	/// <inheritdoc />
@@ -48,10 +60,16 @@ public class NGramTokenFilterDescriptor
 
 		int? INGramTokenFilter.MinGram { get; set; }
 
-		/// <inheritdoc />
+		bool? INGramTokenFilter.PreserveOriginal { get; set; }
+
+		/// <inheritdoc cref="INGramTokenFilter.MinGram" />
 		public NGramTokenFilterDescriptor MinGram(int? minGram) => Assign(minGram, (a, v) => a.MinGram = v);
 
-		/// <inheritdoc />
+		/// <inheritdoc cref="INGramTokenFilter.MaxGram" />
 		public NGramTokenFilterDescriptor MaxGram(int? maxGram) => Assign(maxGram, (a, v) => a.MaxGram = v);
+
+		/// <inheritdoc cref="INGramTokenFilter.PreserveOriginal" />
+		public NGramTokenFilterDescriptor PreserveOriginal(bool? preserveOriginal = true) =>
+			Assign(preserveOriginal, (a, v) => a.PreserveOriginal = v);
 	}
 }

tests/Tests/Analysis/TokenFilters/TokenFilterTests.cs

@@ -489,6 +489,18 @@ public class NGramTests : TokenFilterAssertionBase<NGramTests>
 			public override string Name => "ngram";
 		}
 
+		[SkipVersion("<7.8.0", "PreserveOriginal introduced in 7.8.0")]
+		public class NGramPreserveOriginalTests : TokenFilterAssertionBase<NGramPreserveOriginalTests>
+		{
+			public override FuncTokenFilters Fluent => (n, tf) => tf
+				.NGram(n, t => t.MinGram(3).MaxGram(4).PreserveOriginal());
+
+			public override ITokenFilter Initializer => new NGramTokenFilter { MinGram = 3, MaxGram = 4, PreserveOriginal = true };
+
+			public override object Json => new { type = "ngram", min_gram = 3, max_gram = 4, preserve_original = true };
+			public override string Name => "ngrampo";
+		}
+
 		public class PatternCaptureTests : TokenFilterAssertionBase<PatternCaptureTests>
 		{
 			public override FuncTokenFilters Fluent => (n, tf) => tf