Add expression support for median aggregation.

Original Pull Request: #4515

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/spel/MethodReferenceNode.java

@@ -231,6 +231,8 @@ public class MethodReferenceNode extends ExpressionNode {
 				.mappingParametersTo("n", "input"));
 		map.put("percentile", mapArgRef().forOperator("$percentile") //
 				.mappingParametersTo("input", "p", "method"));
+		map.put("median", mapArgRef().forOperator("$median") //
+				.mappingParametersTo("input", "method"));
 
 		// TYPE OPERATORS
 		map.put("type", singleArgRef().forOperator("$type"));

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java

@@ -1269,6 +1269,16 @@ void shouldRenderPercentile() {
 			.isEqualTo("{ $percentile : { input : \"$score\", p : [0.4, 0.85], method : \"approximate\" }}");
 	}
 
+	@Test // GH-4472
+	void shouldRenderMedian() {
+
+		assertThat(transform("median(new String[]{\"$scoreOne\", \"$scoreTwo\" }, \"approximate\")"))
+				.isEqualTo("{ $median : { input : [\"$scoreOne\", \"$scoreTwo\"], method : \"approximate\" }}");
+
+		assertThat(transform("median(score, \"approximate\")"))
+				.isEqualTo("{ $median : { input : \"$score\", method : \"approximate\" }}");
+	}
+
 	private Document transform(String expression, Object... params) {
 		return (Document) transformer.transform(expression, Aggregation.DEFAULT_CONTEXT, params);
 	}