@MatrixVariable resolvers for WebFlux

The information was already parsed and available in a request attribute
but until now there were no argument resolvers to expose it.

Issue: SPR-16005

spring-web/src/test/java/org/springframework/web/method/MvcAnnotationPredicates.java

@@ -22,6 +22,7 @@ import java.util.function.Predicate;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.MatrixVariable;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
@@ -61,6 +62,9 @@ public class MvcAnnotationPredicates {
 		return new RequestPartPredicate();
 	}
 
+	public static MatrixVariablePredicate matrixAttribute() {
+		return new MatrixVariablePredicate();
+	}
 
 	// Method predicates
 
@@ -305,4 +309,40 @@ public class MvcAnnotationPredicates {
 		}
 	}
 
+	public static class MatrixVariablePredicate implements Predicate<MethodParameter> {
+
+		private String name;
+
+		private String pathVar;
+
+
+		public MatrixVariablePredicate name(String name) {
+			this.name = name;
+			return this;
+		}
+
+		public MatrixVariablePredicate noName() {
+			this.name = "";
+			return this;
+		}
+
+		public MatrixVariablePredicate pathVar(String name) {
+			this.pathVar = name;
+			return this;
+		}
+
+		public MatrixVariablePredicate noPathVar() {
+			this.pathVar = ValueConstants.DEFAULT_NONE;
+			return this;
+		}
+
+		@Override
+		public boolean test(MethodParameter parameter) {
+			MatrixVariable annotation = parameter.getParameterAnnotation(MatrixVariable.class);
+			return annotation != null &&
+					(this.name == null || this.name.equalsIgnoreCase(annotation.name())) &&
+					(this.pathVar == null || this.pathVar.equalsIgnoreCase(annotation.pathVar()));
+		}
+	}
+
 }