samples: Adds tracing sample for cloud spanner (googleapis#1002)

* Add tracing

* Fixes

* tracing

* Add tracing sample

* Revert Quickstart

* Revert Spanner sample

* Address review comments

* Fix style violations

samples/snippets/src/main/java/com/example/spanner/TracingSample.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2018 Google 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.example.spanner;
+
+import com.google.cloud.spanner.DatabaseClient;
+import com.google.cloud.spanner.DatabaseId;
+import com.google.cloud.spanner.ResultSet;
+import com.google.cloud.spanner.Spanner;
+import com.google.cloud.spanner.SpannerOptions;
+import com.google.cloud.spanner.Statement;
+
+import io.opencensus.common.Scope;
+import io.opencensus.contrib.grpc.metrics.RpcViews;
+import io.opencensus.contrib.zpages.ZPageHandlers;
+import io.opencensus.exporter.stats.stackdriver.StackdriverStatsExporter;
+import io.opencensus.exporter.trace.stackdriver.StackdriverExporter;
+import io.opencensus.trace.Tracing;
+import io.opencensus.trace.samplers.Samplers;
+
+import java.util.Arrays;
+
+/**
+ * This sample demonstrates how to enable opencensus tracing and stats in cloud spanner client.
+ */
+public class TracingSample {
+
+  private static final String SAMPLE_SPAN = "CloudSpannerSample";
+
+  public static void main(String[] args) throws Exception {
+    if (args.length != 2) {
+      System.err.println("Usage: TracingSample <instance_id> <database_id>");
+      return;
+    }
+    SpannerOptions options = SpannerOptions.newBuilder().build();
+    Spanner spanner = options.getService();
+
+    // Installs a handler for /tracez page.
+    ZPageHandlers.startHttpServerAndRegisterAll(8080);
+    // Installs an exporter for stack driver traces.
+    StackdriverExporter.createAndRegister();
+    Tracing.getExportComponent().getSampledSpanStore().registerSpanNamesForCollection(
+        Arrays.asList(SAMPLE_SPAN));
+
+    // Installs an exporter for stack driver stats.
+    StackdriverStatsExporter.createAndRegister();
+    RpcViews.registerAllCumulativeViews();
+
+    // Name of your instance & database.
+    String instanceId = args[0];
+    String databaseId = args[1];
+    try {
+      // Creates a database client
+      DatabaseClient dbClient = spanner.getDatabaseClient(DatabaseId.of(
+          options.getProjectId(), instanceId, databaseId));
+      // Queries the database
+      try (Scope ss = Tracing.getTracer()
+          .spanBuilderWithExplicitParent(SAMPLE_SPAN, null)
+          .setSampler(Samplers.alwaysSample())
+          .startScopedSpan()) {
+        ResultSet resultSet = dbClient.singleUse().executeQuery(Statement.of("SELECT 1"));
+
+        System.out.println("\n\nResults:");
+        // Prints the results
+        while (resultSet.next()) {
+          System.out.printf("%d\n\n", resultSet.getLong(0));
+        }
+      }
+    } finally {
+      // Closes the client which will free up the resources used
+      spanner.close();
+    }
+  }
+
+}