No-op implementations for Propagator, Setter, and Getter (#118)

Add no-op implementations for Propagator and its Setter and Getter.

micrometer-tracing/src/main/java/io/micrometer/tracing/propagation/Propagator.java

@@ -19,6 +19,7 @@
 import io.micrometer.tracing.Span;
 import io.micrometer.tracing.TraceContext;
 
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -36,6 +37,29 @@
  */
 public interface Propagator {
 
+    /**
+     * A noop implementation.
+     * <p>
+     * This implementation could be used with sender/receiver that do not need any
+     * propagation. e.g. database access
+     */
+    Propagator NOOP = new Propagator() {
+        @Override
+        public List<String> fields() {
+            return Collections.emptyList();
+        }
+
+        @Override
+        public <C> void inject(TraceContext context, C carrier, Setter<C> setter) {
+
+        }
+
+        @Override
+        public <C> Span.Builder extract(C carrier, Getter<C> getter) {
+            return Span.Builder.NOOP;
+        }
+    };
+
     /**
      * @return collection of headers that contain tracing information
      */
@@ -81,6 +105,13 @@ public interface Propagator {
      */
     interface Setter<C> {
 
+        /**
+         * A noop implementation.
+         */
+        @SuppressWarnings("rawtypes")
+        Setter NOOP = (carrier, key, value) -> {
+        };
+
         /**
          * Replaces a propagated field with the given value.
          *
@@ -110,6 +141,12 @@ interface Setter<C> {
      */
     interface Getter<C> {
 
+        /**
+         * A noop implementation.
+         */
+        @SuppressWarnings("rawtypes")
+        Getter NOOP = (carrier, key) -> null;
+
         /**
          * Returns the first value of the given propagation {@code key} or returns
          * {@code null}.