add resource on tracer (#178)

* add resource on tracer

* empty->EMPTY

api/src/main/java/openconsensus/resource/Resource.java

@@ -53,8 +53,15 @@ public abstract class Resource {
   private static final Map<String, String> ENV_LABEL_MAP =
       parseResourceLabels(System.getenv(OC_RESOURCE_LABELS_ENV));
 
+  private static final Resource EMPTY =
+      new AutoValue_Resource(Collections.<String, String>emptyMap());
+
   Resource() {}
 
+  public static Resource getEmpty() {
+    return EMPTY;
+  }
+
   /**
    * Returns a map of labels that describe the resource.
    *

api/src/main/java/openconsensus/trace/NoopTrace.java

@@ -25,6 +25,7 @@ import openconsensus.context.Scope;
 import openconsensus.context.propagation.BinaryFormat;
 import openconsensus.context.propagation.TextFormat;
 import openconsensus.internal.Utils;
+import openconsensus.resource.Resource;
 
 /**
  * No-op implementations of trace classes.
@@ -91,6 +92,16 @@ public final class NoopTrace {
       Utils.checkNotNull(spanData, "spanData");
     }
 
+    @Override
+    public void setResource(Resource resource) {
+      // do nothing
+    }
+
+    @Override
+    public Resource getResource() {
+      return Resource.getEmpty();
+    }
+
     @Override
     public BinaryFormat<SpanContext> getBinaryFormat() {
       return BINARY_FORMAT;

api/src/main/java/openconsensus/trace/Tracer.java

@@ -22,6 +22,7 @@ import javax.annotation.Nullable;
 import openconsensus.context.Scope;
 import openconsensus.context.propagation.BinaryFormat;
 import openconsensus.context.propagation.TextFormat;
+import openconsensus.resource.Resource;
 
 /**
  * Tracer is a simple, thin class for {@link Span} creation and in-process context interaction.
@@ -330,6 +331,23 @@ public abstract class Tracer {
   public abstract SpanBuilder spanBuilderWithRemoteParent(
       String spanName, @Nullable SpanContext remoteParentSpanContext);
 
+  /**
+   * Sets the {@link Resource} to be associated with all {@link Span} and {@link SpanData} objects
+   * recorded by this {@link Tracer}.
+   *
+   * @param resource Resource to be associated with all {@link Span} and {@link SpanData} objects.
+   */
+  public abstract void setResource(Resource resource);
+
+  /**
+   * Gets the {@link Resource} that is associating with all the {@link Span} and {@link SpanData}
+   * objects recorded by this {@link Tracer}.
+   *
+   * @return {@link Resource} that is associating with all {@link Span} and {@link SpanData}
+   *     objects.
+   */
+  public abstract Resource getResource();
+
   /**
    * Records a {@link SpanData}. This API allows to send a pre-populated span object to the
    * exporter. Sampling and recording decisions as well as other collection optimizations is a