convert PlaceholderFactory to interface (#327)

Changes the PlaceholderFactory class to be an interface
instead. This should give us more flexibility later on if
needed.

Also does some cleanup:

* Remove unused NoopPlaceholderId class.
* Specify registry when constructing a PlaceholderId rather
    then when invoking resolveToId.
* Remove tags method from PlaceholderId interface, which
    was a legacy from when the interface extended Id.

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

@@ -18,7 +18,6 @@
 import com.netflix.spectator.api.Id;
 import com.netflix.spectator.api.Measurement;
 import com.netflix.spectator.api.Meter;
-import com.netflix.spectator.api.Registry;
 
 import java.util.function.Function;
 
@@ -28,31 +27,28 @@
  */
 abstract class AbstractDefaultPlaceholderMeter<T extends Meter> implements Meter {
   private final PlaceholderId id;
-  private final Registry registry;
   private final Function<Id, T> meterResolver;
 
   /**
    * 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, Registry registry, Function<Id, T> meterResolver) {
+  AbstractDefaultPlaceholderMeter(PlaceholderId id, Function<Id, T> meterResolver) {
     this.id = id;
-    this.registry = registry;
     this.meterResolver = meterResolver;
   }
 
   /**
    * Resolve the dynamic id to the current metric instance.
    */
   protected final T resolveToCurrentMeter() {
-    return meterResolver.apply(id.resolveToId(registry));
+    return meterResolver.apply(id());
   }
 
   @Override
   public final Id id() {
-    return resolveToCurrentMeter().id();
+    return id.resolveToId();
   }
 
   @Override

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

@@ -31,7 +31,7 @@ class DefaultPlaceholderCounter extends AbstractDefaultPlaceholderMeter<Counter>
    * @param registry the registry to use to instantiate the individual counters
    */
   DefaultPlaceholderCounter(PlaceholderId id, Registry registry) {
-    super(id, registry, registry::counter);
+    super(id, registry::counter);
   }
 
   @Override

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

@@ -32,7 +32,7 @@ class DefaultPlaceholderDistributionSummary extends AbstractDefaultPlaceholderMe
    * @param registry the registry to use to instantiate the individual distribution summaries
    */
   DefaultPlaceholderDistributionSummary(PlaceholderId id, Registry registry) {
-    super(id, registry, registry::distributionSummary);
+    super(id, registry::distributionSummary);
   }
 
   @Override

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

@@ -0,0 +1,65 @@
+/**
+ * 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.Counter;
+import com.netflix.spectator.api.DistributionSummary;
+import com.netflix.spectator.api.Registry;
+import com.netflix.spectator.api.Timer;
+
+/**
+ * Factory for creating instances of activity based meters with placeholders in the
+ * identifiers so that the final id can be resolved when the activity takes place.
+ */
+final class DefaultPlaceholderFactory implements PlaceholderFactory {
+
+  private final Registry registry;
+
+  /**
+   * Create a new instance of the factory.
+   *
+   * @param registry
+   *      the registry to use when creating ids
+   */
+  DefaultPlaceholderFactory(Registry registry) {
+    this.registry = registry;
+  }
+
+  @Override
+  public PlaceholderId createId(String name) {
+    return new DefaultPlaceholderId(name, registry);
+  }
+
+  @Override
+  public PlaceholderId createId(String name, Iterable<TagFactory> tagFactories) {
+    return DefaultPlaceholderId.createWithFactories(name, tagFactories, registry);
+  }
+
+  @Override
+  public Counter counter(PlaceholderId id) {
+    return new DefaultPlaceholderCounter(id, registry);
+  }
+
+  @Override
+  public DistributionSummary distributionSummary(PlaceholderId id) {
+    return new DefaultPlaceholderDistributionSummary(id, registry);
+  }
+
+  @Override
+  public Timer timer(PlaceholderId id) {
+    return new DefaultPlaceholderTimer(id, registry);
+  }
+}

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

@@ -79,6 +79,7 @@ Collection<TagFactory> asCollection() {
 
   private final String name;
   private final Collection<TagFactory> tagFactories;
+  private final Registry registry;
 
   /**
    * Creates a new id with the specified name and collection of factories.
@@ -87,52 +88,48 @@ Collection<TagFactory> asCollection() {
    *      the name of the new id
    * @param tagFactories
    *      the possibly empty collection of factories to be attached to the new id
+   * @param registry
+   *      the registry to use when resolving to an {@link Id}
    * @return
    *      the newly created id
    */
-  static DefaultPlaceholderId createWithFactories(String name, Iterable<TagFactory> tagFactories) {
+  static DefaultPlaceholderId createWithFactories(String name, Iterable<TagFactory> tagFactories, Registry registry) {
     if (tagFactories == null) {
-      return new DefaultPlaceholderId(name);
+      return new DefaultPlaceholderId(name, registry);
     } else {
       FactorySorterAndDeduplicator sorter = new FactorySorterAndDeduplicator(tagFactories);
 
-      return new DefaultPlaceholderId(name, sorter.asCollection());
+      return new DefaultPlaceholderId(name, sorter.asCollection(), registry);
     }
   }
 
   /**
    * Constructs a new id with the specified name and no associated tag factories.
    */
-  DefaultPlaceholderId(String name) {
-    this(name, Collections.emptyList());
+  DefaultPlaceholderId(String name, Registry registry) {
+    this(name, Collections.emptyList(), registry);
   }
 
   /**
    * Constructs a new id with the specified name and tag factories.
-   *
-   * @param name
+   *  @param name
    *      the name of the new id
    * @param tagFactories
-   *      the possibly empty, immutable collection of tag factories to use
+   *      the possibly empty collection of factories to be attached to the new id
+   * @param registry
+   *      the registry to use when resolving to an {@link Id}
    */
-  private DefaultPlaceholderId(String name, Collection<TagFactory> tagFactories) {
+  private DefaultPlaceholderId(String name, Collection<TagFactory> tagFactories, Registry registry) {
     this.name = Preconditions.checkNotNull(name, "name");
     this.tagFactories = tagFactories;
+    this.registry = registry;
   }
 
   @Override
   public String name() {
     return name;
   }
 
-  @Override
-  public Iterable<Tag> tags() {
-    return tagFactories.stream()
-      .map(TagFactory::createTag)
-      .filter(tag -> tag != null)
-      .collect(Collectors.toList());
-  }
-
   @Override
   public DefaultPlaceholderId withTag(String k, String v) {
     return withTagFactory(new ConstantTagFactory(new BasicTag(k, v)));
@@ -157,7 +154,7 @@ public DefaultPlaceholderId withTags(Map<String, String> tags) {
   @Override
   public DefaultPlaceholderId withTagFactory(TagFactory factory) {
     if (tagFactories.isEmpty()) {
-      return new DefaultPlaceholderId(name, Collections.singleton(factory));
+      return new DefaultPlaceholderId(name, Collections.singleton(factory), registry);
     } else {
       return createNewId(sorter -> sorter.addFactory(factory));
     }
@@ -169,8 +166,13 @@ public DefaultPlaceholderId withTagFactories(Iterable<TagFactory> factories) {
   }
 
   @Override
-  public Id resolveToId(Registry registry) {
-    return registry.createId(name, tags());
+  public Id resolveToId() {
+    Iterable<Tag> tags = tagFactories.stream()
+            .map(TagFactory::createTag)
+            .filter(tag -> tag != null)
+            .collect(Collectors.toList());
+
+    return registry.createId(name, tags);
   }
 
   @Override
@@ -193,14 +195,7 @@ public int hashCode() {
 
   @Override
   public String toString() {
-    StringBuilder buf = new StringBuilder();
-    buf.append(name);
-    if (!tagFactories.isEmpty()) {
-      for (Tag cur: tags()) {
-        buf.append(":").append(cur.key()).append("=").append(cur.value());
-      }
-    }
-    return buf.toString();
+    return resolveToId().toString();
   }
 
   /**
@@ -216,6 +211,6 @@ private DefaultPlaceholderId createNewId(Consumer<FactorySorterAndDeduplicator>
     FactorySorterAndDeduplicator sorter = new FactorySorterAndDeduplicator(tagFactories);
 
     consumer.accept(sorter);
-    return new DefaultPlaceholderId(name, sorter.asCollection());
+    return new DefaultPlaceholderId(name, sorter.asCollection(), registry);
   }
 }

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

@@ -34,7 +34,7 @@ class DefaultPlaceholderTimer extends AbstractDefaultPlaceholderMeter<Timer> imp
    * @param registry the registry to use to instantiate the individual timers
    */
   DefaultPlaceholderTimer(PlaceholderId id, Registry registry) {
-    super(id, registry, registry::timer);
+    super(id, registry::timer);
   }
 
   @Override