Add nullable to Observation.parentObservation (#3532)

Author: ttddyy

micrometer-observation/src/main/java/io/micrometer/observation/NoopObservation.java

@@ -16,6 +16,7 @@
 package io.micrometer.observation;
 
 import io.micrometer.common.KeyValue;
+import io.micrometer.common.lang.Nullable;
 
 /**
  * No-op implementation of {@link Observation} so that we can disable the instrumentation
@@ -39,12 +40,12 @@ private NoopObservation() {
     }
 
     @Override
-    public Observation contextualName(String contextualName) {
+    public Observation contextualName(@Nullable String contextualName) {
         return this;
     }
 
     @Override
-    public Observation parentObservation(Observation parentObservation) {
+    public Observation parentObservation(@Nullable Observation parentObservation) {
         return this;
     }
 

micrometer-observation/src/main/java/io/micrometer/observation/Observation.java

@@ -302,7 +302,7 @@ static <T extends Context> Observation createNotStarted(ObservationConvention<T>
      * @param parentObservation parent observation to set
      * @return this
      */
-    Observation parentObservation(Observation parentObservation);
+    Observation parentObservation(@Nullable Observation parentObservation);
 
     /**
      * Adds a low cardinality key value. Low cardinality means that this key value will

micrometer-observation/src/main/java/io/micrometer/observation/SimpleObservation.java

@@ -85,7 +85,7 @@ public Observation contextualName(@Nullable String contextualName) {
     }
 
     @Override
-    public Observation parentObservation(Observation parentObservation) {
+    public Observation parentObservation(@Nullable Observation parentObservation) {
         this.context.setParentObservation(parentObservation);
         return this;
     }