Set matching pattern in reactive server observation context

This commit fixes the observation instrumentation for the reactive HTTP
server by setting the best matching pattern determined by the web
framework into the `ServerRequestObservationContext`.

This information is required by the observation convention for creating
the expected `KeyValue` for the matching pattern. Prior to this commit,
the information was missing and resulted in an UNKNOWN key value.

Fixes gh-29422

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/support/RouterFunctionMapping.java

@@ -28,6 +28,7 @@ import org.springframework.http.codec.HttpMessageReader;
 import org.springframework.http.codec.ServerCodecConfigurer;
 import org.springframework.lang.Nullable;
 import org.springframework.util.CollectionUtils;
+import org.springframework.web.filter.reactive.ServerHttpObservationFilter;
 import org.springframework.web.reactive.function.server.HandlerFunction;
 import org.springframework.web.reactive.function.server.RouterFunction;
 import org.springframework.web.reactive.function.server.RouterFunctions;
@@ -170,6 +171,8 @@ public class RouterFunctionMapping extends AbstractHandlerMapping implements Ini
 		PathPattern matchingPattern = (PathPattern) attributes.get(RouterFunctions.MATCHING_PATTERN_ATTRIBUTE);
 		if (matchingPattern != null) {
 			attributes.put(BEST_MATCHING_PATTERN_ATTRIBUTE, matchingPattern);
+			ServerHttpObservationFilter.findObservationContext(serverRequest.exchange())
+					.ifPresent(context -> context.setPathPattern(matchingPattern));
 		}
 		Map<String, String> uriVariables =
 				(Map<String, String>) attributes.get(RouterFunctions.URI_TEMPLATE_VARIABLES_ATTRIBUTE);

spring-webflux/src/main/java/org/springframework/web/reactive/handler/AbstractUrlHandlerMapping.java

@@ -30,6 +30,7 @@ import org.springframework.http.server.PathContainer;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
+import org.springframework.web.filter.reactive.ServerHttpObservationFilter;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.util.pattern.PathPattern;
 
@@ -165,6 +166,8 @@ public abstract class AbstractUrlHandlerMapping extends AbstractHandlerMapping {
 
 		exchange.getAttributes().put(BEST_MATCHING_HANDLER_ATTRIBUTE, handler);
 		exchange.getAttributes().put(BEST_MATCHING_PATTERN_ATTRIBUTE, pattern);
+		ServerHttpObservationFilter.findObservationContext(exchange)
+				.ifPresent(context -> context.setPathPattern(pattern));
 		exchange.getAttributes().put(PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE, pathWithinMapping);
 		exchange.getAttributes().put(URI_TEMPLATE_VARIABLES_ATTRIBUTE, matchInfo.getUriVariables());
 

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/RequestMappingInfoHandlerMapping.java

@@ -38,6 +38,7 @@ import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.util.Assert;
 import org.springframework.util.MultiValueMap;
 import org.springframework.web.bind.annotation.RequestMethod;
+import org.springframework.web.filter.reactive.ServerHttpObservationFilter;
 import org.springframework.web.method.HandlerMethod;
 import org.springframework.web.reactive.HandlerMapping;
 import org.springframework.web.reactive.result.condition.NameValueExpression;
@@ -139,6 +140,8 @@ public abstract class RequestMappingInfoHandlerMapping extends AbstractHandlerMe
 
 		exchange.getAttributes().put(BEST_MATCHING_HANDLER_ATTRIBUTE, handlerMethod);
 		exchange.getAttributes().put(BEST_MATCHING_PATTERN_ATTRIBUTE, bestPattern);
+		ServerHttpObservationFilter.findObservationContext(exchange)
+				.ifPresent(context -> context.setPathPattern(bestPattern));
 		exchange.getAttributes().put(URI_TEMPLATE_VARIABLES_ATTRIBUTE, uriVariables);
 		exchange.getAttributes().put(MATRIX_VARIABLES_ATTRIBUTE, matrixVariables);