Add an API that does not require user to write a project_id. (#589)

* Add an API that does not require user to write a project_id.

* Resolve comments.

* Fix javadoc example.

Author: Bogdan Drutu

exporters/trace_stackdriver/README.md

@@ -22,15 +22,6 @@ In order to be able to push your traces to [Stackdriver Trace][stackdriver-trace
 
 These steps enable the API but don't require that your app is hosted on Google Cloud Platform.
 
-### Authentication
-
-This exporter uses [google-cloud-java](https://github.com/GoogleCloudPlatform/google-cloud-java),
-for details see [here](https://github.com/GoogleCloudPlatform/google-cloud-java#authentication).
-
-### Java Versions
-
-Java 7 or above is required for using this exporter.
-
 ### Hello "Stackdriver Trace"
 
 #### Add the dependencies to your project
@@ -53,21 +44,50 @@ For Maven add to your `pom.xml`:
 ```
 
 For Gradle add to your dependencies:
-```gradle
+```groovy
 compile 'io.opencensus:opencensus-exporter-trace-stackdriver:0.6.0'
 runtime 'io.opencensus:opencensus-impl:0.6.0'
 ```
 
 #### Register the exporter
+
+This uses the default configuration for authentication and project ID.
+
 ```java
 public class MyMainClass {
   public static void main(String[] args) throws Exception {
-    StackdriverExporter.createAndRegister("MyStackdriverProjectId");
+    StackdriverExporter.createAndRegister();
     // ...
   }
 }
 ```
 
+#### Authentication
+
+This exporter uses [google-cloud-java](https://github.com/GoogleCloudPlatform/google-cloud-java),
+for details about how to configure the authentication see [here](https://github.com/GoogleCloudPlatform/google-cloud-java#authentication).
+
+If you prefer to manually set the credentials use:
+```
+StackdriverExporter.createAndRegisterWithCredentialsAndProjectId(
+    new GoogleCredentials(new AccessToken(accessToken, expirationTime)),
+    "MyStackdriverProjectId");
+```
+
+#### Specifying a Project ID
+
+This exporter uses [google-cloud-java](https://github.com/GoogleCloudPlatform/google-cloud-java),
+for details about how to configure the project ID see [here](https://github.com/GoogleCloudPlatform/google-cloud-java#specifying-a-project-id).
+
+If you prefer to manually set the project ID use:
+```
+StackdriverExporter.createAndRegisterWithProjectId("MyStackdriverProjectId");
+```
+
+#### Java Versions
+
+Java 7 or above is required for using this exporter.
+
 ## FAQ
 ### Why do I not see some trace events in Stackdriver?
 The current Stackdriver Trace exporter is implemented using the [v1 API][stackdriver-v1-api-url]

exporters/trace_stackdriver/src/main/java/io/opencensus/exporter/trace/stackdriver/StackdriverExporter.java

@@ -20,13 +20,12 @@
 
 import com.google.auth.Credentials;
 import com.google.auth.oauth2.GoogleCredentials;
+import com.google.cloud.ServiceOptions;
 import com.google.common.annotations.VisibleForTesting;
 import io.opencensus.trace.Tracing;
 import io.opencensus.trace.export.SpanExporter;
 import io.opencensus.trace.export.SpanExporter.Handler;
 import java.io.IOException;
-import java.util.Collections;
-import java.util.List;
 import javax.annotation.concurrent.GuardedBy;
 
 /**
@@ -36,29 +35,30 @@
  *
  * <pre><code>
  *   public static void main(String[] args) {
- *     StackdriverExporter.createAndRegister("MyStackdriverProjectId");
+ *     StackdriverExporter.createAndRegisterWithProjectId("MyStackdriverProjectId");
  *     ... // Do work.
  *   }
  * </code></pre>
  */
 public final class StackdriverExporter {
+
   private static final String REGISTER_NAME = StackdriverExporter.class.getName();
-  private static final List<String> STACKDRIVER_TRACE_WRITER_SCOPE =
-      Collections.singletonList("https://www.googleapis.com/auth/trace.append");
   private static final Object monitor = new Object();
 
   @GuardedBy("monitor")
   private static Handler handler = null;
 
   /**
-   * Creates and registers the Stackdriver Trace exporter to the OpenCensus library. Only one
-   * Stackdriver exporter can be registered at any point.
+   * Creates and registers the Stackdriver Trace exporter to the OpenCensus library for an explicit
+   * project ID and using explicit credentials. Only one Stackdriver exporter can be registered at
+   * any point.
    *
    * @param credentials a credentials used to authenticate API calls.
    * @param projectId the cloud project id.
    * @throws IllegalStateException if a Stackdriver exporter is already registered.
    */
-  public static void createAndRegisterWithCredentials(Credentials credentials, String projectId)
+  public static void createAndRegisterWithCredentialsAndProjectId(Credentials credentials,
+      String projectId)
       throws IOException {
     synchronized (monitor) {
       checkState(handler == null, "Stackdriver exporter is already registered.");
@@ -68,24 +68,55 @@ public static void createAndRegisterWithCredentials(Credentials credentials, Str
   }
 
   /**
-   * Creates and registers the Stackdriver Trace exporter to the OpenCensus library. Only one
-   * Stackdriver exporter can be registered at any point.
+   * Creates and registers the Stackdriver Trace exporter to the OpenCensus library for an explicit
+   * project ID. Only one Stackdriver exporter can be registered at any point.
    *
    * <p>This uses the default application credentials see {@link
-   * GoogleCredentials#getApplicationDefault()}. If you do not have default application credentials
-   * configured use {@link #createAndRegisterWithCredentials(Credentials, String)}.
+   * GoogleCredentials#getApplicationDefault}.
+   *
+   * <p>This is equivalent with:
+   *
+   * <pre>{@code
+   * StackdriverExporter.createAndRegisterWithCredentialsAndProjectId(
+   *     GoogleCredentials.getApplicationDefault(), projectId);
+   * }</pre>
    *
    * @param projectId the cloud project id.
    * @throws IllegalStateException if a Stackdriver exporter is already registered.
    */
-  public static void createAndRegister(String projectId) throws IOException {
+  public static void createAndRegisterWithProjectId(String projectId) throws IOException {
     synchronized (monitor) {
       checkState(handler == null, "Stackdriver exporter is already registered.");
       handler = StackdriverV1ExporterHandler.create(projectId);
       register(Tracing.getExportComponent().getSpanExporter(), handler);
     }
   }
 
+  /**
+   * Creates and registers the Stackdriver Trace exporter to the OpenCensus library. Only one
+   * Stackdriver exporter can be registered at any point.
+   *
+   * <p>This uses the default application credentials see {@link
+   * GoogleCredentials#getApplicationDefault}.
+   *
+   * <p>This uses the default project ID configured see {@link ServiceOptions#getDefaultProjectId}.
+   *
+   * <p>This is equivalent with:
+   *
+   * <pre>{@code
+   * StackdriverExporter.createAndRegisterWithProjectId(ServiceOptions.getDefaultProjectId());
+   * }</pre>
+   *
+   * @throws IllegalStateException if a Stackdriver exporter is already registered.
+   */
+  public static void createAndRegister() throws IOException {
+    synchronized (monitor) {
+      checkState(handler == null, "Stackdriver exporter is already registered.");
+      handler = StackdriverV1ExporterHandler.create(ServiceOptions.getDefaultProjectId());
+      register(Tracing.getExportComponent().getSpanExporter(), handler);
+    }
+  }
+
   /**
    * Registers the {@code StackdriverExporter}.
    *

exporters/trace_stackdriver/src/main/java/io/opencensus/exporter/trace/stackdriver/StackdriverV1ExporterHandler.java

@@ -48,6 +48,7 @@
  * messages.
  */
 final class StackdriverV1ExporterHandler extends SpanExporter.Handler {
+
   private static final String STATUS_CODE = "g.co/status/code";
   private static final String STATUS_DESCRIPTION = "g.co/status/description";