Update Stackdriver Trace exporter to use V2 API. (#808)

* Update Stackdriver Trace exporter to use V2 API.

* Add a span with explicit probability sampling

Author: Bogdan Drutu

build.gradle

@@ -111,7 +111,7 @@ subprojects {
         grpcVersion = '1.7.0'
         guavaVersion = '19.0'
         googleAuthVersion = '0.8.0'
-        googleCloudVersion = '0.26.0-alpha'
+        googleCloudVersion = '0.30.0-beta'
         zipkinReporterVersion = '2.0.0'
 
         libraries = [

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

@@ -61,7 +61,7 @@ public static void createAndRegisterWithCredentialsAndProjectId(
       Credentials credentials, String projectId) throws IOException {
     synchronized (monitor) {
       checkState(handler == null, "Stackdriver exporter is already registered.");
-      handler = StackdriverV1ExporterHandler.createWithCredentials(credentials, projectId);
+      handler = StackdriverV2ExporterHandler.createWithCredentials(credentials, projectId);
       register(Tracing.getExportComponent().getSpanExporter(), handler);
     }
   }
@@ -86,7 +86,7 @@ public static void createAndRegisterWithCredentialsAndProjectId(
   public static void createAndRegisterWithProjectId(String projectId) throws IOException {
     synchronized (monitor) {
       checkState(handler == null, "Stackdriver exporter is already registered.");
-      handler = StackdriverV1ExporterHandler.create(projectId);
+      handler = StackdriverV2ExporterHandler.create(projectId);
       register(Tracing.getExportComponent().getSpanExporter(), handler);
     }
   }
@@ -111,7 +111,7 @@ public static void createAndRegisterWithProjectId(String projectId) throws IOExc
   public static void createAndRegister() throws IOException {
     synchronized (monitor) {
       checkState(handler == null, "Stackdriver exporter is already registered.");
-      handler = StackdriverV1ExporterHandler.create(ServiceOptions.getDefaultProjectId());
+      handler = StackdriverV2ExporterHandler.create(ServiceOptions.getDefaultProjectId());
       register(Tracing.getExportComponent().getSpanExporter(), handler);
     }
   }