Add getters for delegates (#2273)

Fixes gh-2261

spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/async/TraceCallable.java

@@ -75,4 +75,11 @@ public class TraceCallable<V> implements Callable<V> {
 		}
 	}
 
+	/**
+	 * @return delegate {@link Callable}
+	 */
+	public Callable<V> getDelegate() {
+		return delegate;
+	}
+
 }

spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/async/TraceRunnable.java

@@ -72,4 +72,11 @@ public class TraceRunnable implements Runnable {
 		}
 	}
 
+	/**
+	 * @return delegate {@link Runnable}
+	 */
+	public Runnable getDelegate() {
+		return delegate;
+	}
+
 }