Return value of aspect-timed method is Nullable

It can cause issues in Kotlin for this to be incorrectly marked as `@NonNullApi`.

Closes gh-3041

Author: shakuzen

micrometer-core/src/main/java/io/micrometer/core/aop/TimedAspect.java

@@ -24,6 +24,7 @@
 import io.micrometer.core.instrument.Timer;
 import io.micrometer.core.instrument.Metrics;
 import io.micrometer.core.lang.NonNullApi;
+import io.micrometer.core.lang.Nullable;
 import org.aspectj.lang.ProceedingJoinPoint;
 import org.aspectj.lang.annotation.Around;
 import org.aspectj.lang.annotation.Aspect;
@@ -151,6 +152,7 @@ public TimedAspect(MeterRegistry registry, Function<ProceedingJoinPoint, Iterabl
     }
 
     @Around("execution (@io.micrometer.core.annotation.Timed * *.*(..))")
+    @Nullable
     public Object timedMethod(ProceedingJoinPoint pjp) throws Throwable {
         if (shouldSkip.test(pjp)) {
             return pjp.proceed();