Changed isSampled() to Boolean since we could also defer a smapling decision

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/TraceContext.java

@@ -46,8 +46,8 @@ public interface TraceContext {
 	String spanId();
 
 	/**
-	 * @return {@code true} when sampled
+	 * @return {@code true} when sampled, {@code false} when not sampled and {@code null} when sampling decision should be deferred
 	 */
-	boolean isSampled();
+	Boolean isSampled();
 
 }

spring-cloud-sleuth-autoconfigure/src/test/java/org/springframework/cloud/sleuth/autoconfig/NoOpTraceContext.java

@@ -42,7 +42,7 @@ class NoOpTraceContext implements TraceContext {
 	}
 
 	@Override
-	public boolean isSampled() {
+	public Boolean isSampled() {
 		return false;
 	}
 

spring-cloud-sleuth-brave/src/main/java/org/springframework/cloud/sleuth/brave/bridge/BraveTraceContext.java

@@ -52,7 +52,7 @@ class BraveTraceContext implements TraceContext {
 	}
 
 	@Override
-	public boolean isSampled() {
+	public Boolean isSampled() {
 		return this.traceContext.sampled();
 	}
 

spring-cloud-sleuth-otel/src/main/java/org/springframework/cloud/sleuth/otel/bridge/OtelTraceContext.java

@@ -100,7 +100,7 @@ class OtelTraceContext implements TraceContext {
 	}
 
 	@Override
-	public boolean isSampled() {
+	public Boolean isSampled() {
 		return this.delegate.isSampled();
 	}