Ignore NullObservation in ObservationValidator

NullObservation is a special case for context propagation. It's not
fully an Observation since it is only for scope handling which can
happen outside of "normal" Observations, therefore we not necessarily
need to validate it.

Closes micrometer-metricsgh-5388

micrometer-observation-test/src/main/java/io/micrometer/observation/tck/ObservationValidator.java

@@ -16,6 +16,7 @@
 package io.micrometer.observation.tck;
 
 import io.micrometer.common.lang.Nullable;
+import io.micrometer.observation.NullObservation.NullContext;
 import io.micrometer.observation.Observation.Context;
 import io.micrometer.observation.Observation.Event;
 import io.micrometer.observation.ObservationHandler;
@@ -47,7 +48,7 @@ class ObservationValidator implements ObservationHandler<Context> {
     }
 
     ObservationValidator(Consumer<ValidationResult> consumer) {
-        this(consumer, context -> true);
+        this(consumer, context -> !(context instanceof NullContext));
     }
 
     ObservationValidator(Consumer<ValidationResult> consumer, Predicate<Context> supportsContextPredicate) {

micrometer-observation-test/src/test/java/io/micrometer/observation/tck/ObservationValidatorTests.java

@@ -15,6 +15,7 @@
  */
 package io.micrometer.observation.tck;
 
+import io.micrometer.observation.NullObservation;
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.Observation.Event;
 import io.micrometer.observation.Observation.Scope;
@@ -253,4 +254,9 @@ void startErrorErrorStopShouldBeValid() {
         Observation.start("test", registry).error(new RuntimeException()).error(new RuntimeException()).stop();
     }
 
+    @Test
+    void nullObservationShouldBeIgnored() {
+        new NullObservation(registry).openScope();
+    }
+
 }

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

@@ -28,7 +28,7 @@
 public class NullObservation extends SimpleObservation {
 
     public NullObservation(ObservationRegistry registry) {
-        super("null", registry, new Context());
+        super("null", registry, new NullContext());
     }
 
     @Override
@@ -66,4 +66,16 @@ public Observation start() {
         return this;
     }
 
+    /**
+     * A special {@link Observation.Context} that should be used only in
+     * {@link NullObservation} in special cases where clearing of scopes is important. Its
+     * only purpose is to make scenarios through {@link NullObservation} distinguishable
+     * from "normal" {@link Observation Observations}.
+     *
+     * @since 1.14.0
+     */
+    public static class NullContext extends Context {
+
+    }
+
 }