add MdcTagFactory (#328)

TagFactory implementation that pulls values from the
slf4j MDC data.

spectator-ext-placeholders/build.gradle

@@ -1,3 +1,8 @@
 dependencies {
   compileApi project(':spectator-api')
+
+  // The MdcTagFactoryTest requires a logging implementation that supports the MDC
+  // functionality, which the slf4j-nop (default) and slf4j-simple implementations do not.
+  testCompile "org.slf4j:slf4j-log4j12"
+  testCompile "log4j:log4j"
 }

spectator-ext-placeholders/src/main/java/com/netflix/spectator/placeholders/AbstractDefaultPlaceholderMeter.java

@@ -31,7 +31,8 @@ abstract class AbstractDefaultPlaceholderMeter<T extends Meter> implements Meter
 
   /**
    * Creates a new dynamic meter.
-   *  @param id the dynamic id for the meter
+   *
+   * @param id the dynamic id for the meter
    * @param meterResolver the function to map a resolved id to concrete metric
    */
   AbstractDefaultPlaceholderMeter(PlaceholderId id, Function<Id, T> meterResolver) {

spectator-ext-placeholders/src/main/java/com/netflix/spectator/placeholders/MdcTagFactory.java

@@ -0,0 +1,55 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.netflix.spectator.placeholders;
+
+import com.netflix.spectator.api.BasicTag;
+import com.netflix.spectator.api.Tag;
+import com.netflix.spectator.impl.Preconditions;
+import org.slf4j.MDC;
+
+/**
+ * A TagFactory implementation that extracts information from the Sl4fj MDC data.
+ * If the MDC for the current thread has no value associated with the specified
+ * name at the time that the createTag method is invoked, then that method will
+ * return null, which will result in the tag being omitted.
+ */
+public class MdcTagFactory implements TagFactory {
+
+  private final String name;
+
+  /**
+   * Construct a new instance that will return a Tag with the MDC value associated
+   * with the specified name at the time that the createTag method is invoked.
+   *
+   * @param name
+   *        the non-null name of the MDC value to use for the tag
+   */
+  public MdcTagFactory(String name) {
+    this.name = Preconditions.checkNotNull(name, "name");
+  }
+
+  @Override
+  public String name() {
+    return name;
+  }
+
+  @Override
+  public Tag createTag() {
+    String value = MDC.get(name);
+
+    return value != null ? new BasicTag(name, value) : null;
+  }
+}

spectator-ext-placeholders/src/test/java/com/netflix/spectator/placeholders/MdcTagFactoryTest.java

@@ -0,0 +1,68 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.netflix.spectator.placeholders;
+
+import com.netflix.spectator.api.BasicTag;
+import com.netflix.spectator.api.Tag;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.slf4j.MDC;
+
+/**
+ * Unit tests for the MdcTagFactory class.
+ */
+@RunWith(JUnit4.class)
+public class MdcTagFactoryTest {
+
+  @Test(expected = NullPointerException.class)
+  public void testNullName() {
+    new MdcTagFactory(null);
+  }
+
+  @Test
+  public void testNameFromKey() throws Exception {
+    String expected = "factoryName";
+    TagFactory factory = new MdcTagFactory(expected);
+
+    Assert.assertEquals(expected, factory.name());
+  }
+
+  @Test
+  public void testNoValueInMdc() {
+    TagFactory factory = new MdcTagFactory("unused");
+
+    Assert.assertNull(factory.createTag());
+  }
+
+  @Test
+  public void testValueInMdc() {
+    String mdcName = "key";
+    String expectedValue = "value";
+    Tag expectedTag = new BasicTag(mdcName, expectedValue);
+    TagFactory factory = new MdcTagFactory("key");
+
+    try (MDC.MDCCloseable closeable = MDC.putCloseable(mdcName, expectedValue)) {
+      Tag actualTag = factory.createTag();
+
+      Assert.assertEquals(expectedTag, actualTag);
+    }
+
+    // Make sure that the factory returns null after the MDC has been cleaned up.
+    Assert.assertNull(factory.createTag());
+  }
+}