fix spring-webflux cast to PathPattern throws ClassCastException (ope…

…n-telemetry#6872)

when
ServerWebExchange.getAttributes().put(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE,
"path string type").
opentelemetry javaagent will throw exception.

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instrumentation/spring/spring-webflux-5.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/spring/webflux/server/WebfluxSingletons.java

@@ -41,9 +41,14 @@ public static Instrumenter<Object, Void> instrumenter() {
 
   public static HttpRouteGetter<ServerWebExchange> httpRouteGetter() {
     return (context, exchange) -> {
-      PathPattern bestPattern =
-          exchange.getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE);
-      return bestPattern == null ? null : bestPattern.getPatternString();
+      Object bestPatternObj = exchange.getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE);
+      if (bestPatternObj == null) {
+        return null;
+      }
+      if (bestPatternObj instanceof PathPattern) {
+        return ((PathPattern) bestPatternObj).getPatternString();
+      }
+      return bestPatternObj.toString();
     };
   }