Add support for multiplexer token filter (elastic/elasticsearch#31208)

src/Nest/Analysis/TokenFilters/TokenFilters.cs

@@ -317,5 +317,12 @@ public TokenFiltersDescriptor IcuNormalization(string name, Func<IcuNormalizatio
 		/// </summary>
 		public TokenFiltersDescriptor IcuTransform(string name, Func<IcuTransformTokenFilterDescriptor, IIcuTransformTokenFilter> selector) =>
 			Assign(name, selector.Invoke(new IcuTransformTokenFilterDescriptor()));
+
+		/// <summary>
+		///  A token filter of type multiplexer will emit multiple tokens at the same position, each version of the token
+		/// having been run through a different filter. Identical output tokens at the same position will be removed.
+		/// </summary>
+		public TokenFiltersDescriptor Multiplexer(string name, Func<MultiplexerTokenFilterDescriptor, IMultiplexerTokenFilter> selector) =>
+			Assign(name, selector.Invoke(new MultiplexerTokenFilterDescriptor()));
 	}
 }

src/Tests/Tests/Analysis/TokenFilters/TokenFilterTests.cs

@@ -1,4 +1,5 @@
 using System;
+using Elastic.Xunit.XunitPlumbing;
 using Nest;
 using Tests.Framework;
 
@@ -878,5 +879,28 @@ public class PhoneticTests : TokenFilterAssertionBase<PhoneticTests>
 			};
 
 		}
+
+		[SkipVersion("<6.4.0", "Introduced in 6.4.0")]
+		public class MultiplexerTests : TokenFilterAssertionBase<PhoneticTests>
+		{
+			public override string Name => "multiplexer";
+			public override object Json => new
+			{
+				filters = new[]{"lowercase", "lowercase, porter_stem"},
+				preserve_original = true
+			};
+
+			public override ITokenFilter Initializer => new MultiplexerTokenFilter
+			{
+				Filters = new[] {"lowercase", "lowercase, porter_stem"},
+				PreserveOriginal = true
+			};
+
+			public override FuncTokenFilters Fluent => (n, tf) => tf
+				.Multiplexer(n, t => t
+					.Filters("lowercase", "lowercase, porter_stem")
+					.PreserveOriginal()
+				);
+		}
 	}
 }