Avoid NPE when creating method tag for WebFlux req with non-standard method

Previously, a NullPointerException would occur when WebFluxTags
attempted to create a method Tag for a request with a non-standard
method.

This commit updates WebFluxTags to use getMethodValue(), which will
never return null, rather than getMethod(), which may return null,
when determining the tag's value for the given request.

Closes gh-13596

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTags.java

@@ -55,7 +55,7 @@ public final class WebFluxTags {
 	 * @return the method tag whose value is a capitalized method (e.g. GET).
 	 */
 	public static Tag method(ServerWebExchange exchange) {
-		return Tag.of("method", exchange.getRequest().getMethod().toString());
+		return Tag.of("method", exchange.getRequest().getMethodValue());
 	}
 
 	/**

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTagsTests.java

@@ -21,12 +21,16 @@ import org.junit.Before;
 import org.junit.Test;
 
 import org.springframework.http.HttpStatus;
+import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.mock.http.server.reactive.MockServerHttpRequest;
 import org.springframework.mock.web.server.MockServerWebExchange;
 import org.springframework.web.reactive.HandlerMapping;
+import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.util.pattern.PathPatternParser;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
 
 /**
  * Tests for {@link WebFluxTags}.
@@ -80,4 +84,14 @@ public class WebFluxTagsTests {
 		assertThat(tag.getValue()).isEqualTo("UNKNOWN");
 	}
 
+	@Test
+	public void methodTagToleratesNonStandardHttpMethods() {
+		ServerWebExchange exchange = mock(ServerWebExchange.class);
+		ServerHttpRequest request = mock(ServerHttpRequest.class);
+		given(exchange.getRequest()).willReturn(request);
+		given(request.getMethodValue()).willReturn("CUSTOM");
+		Tag tag = WebFluxTags.method(exchange);
+		assertThat(tag.getValue()).isEqualTo("CUSTOM");
+	}
+
 }