Add example for tracing (#379)

* Add example for tracing

* style fix

* style fix

Author: mayurkale22

examples/trace/multi-span-tracing.js

@@ -0,0 +1,67 @@
+/**
+ * Copyright 2019, OpenCensus Authors
+ *
+ * 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.
+ */
+
+/** Example showing how to directly create a child span and add annotations. */
+const tracing = require('@opencensus/nodejs');
+const { ZipkinTraceExporter } = require('@opencensus/exporter-zipkin');
+
+// 1. Get the global singleton Tracer object
+// 2. Configure 100% sample rate, otherwise, few traces will be sampled.
+const tracer = tracing.start({ samplingRate: 1 }).tracer;
+
+// 3. Configure exporter to export traces to Zipkin.
+tracer.registerSpanEventListener(new ZipkinTraceExporter({
+  url: 'http://localhost:9411/api/v2/spans',
+  serviceName: 'node.js-quickstart'
+}));
+
+function main () {
+  // 4. Create a span. A span must be closed.
+  // For any of the web frameworks for which we provide built-in plugins (http,
+  // grpc, mongodb etc), a root span is automatically started whenever an
+  // incoming request is received (in other words, all middleware already runs
+  // within a root span).
+  tracer.startRootSpan({ name: 'main' }, rootSpan => {
+    for (let i = 0; i < 10; i++) {
+      doWork();
+    }
+
+    // Be sure to call rootSpan.end().
+    rootSpan.end();
+  });
+
+  // Add short delay to allow data to export.
+  setTimeout(() => {
+    console.log('done.');
+  }, 2000);
+}
+
+function doWork () {
+  // 5. Start another span. In this example, the main method already started a
+  // span, so that'll be the parent span, and this will be a child span.
+  const span = tracer.startChildSpan('doWork');
+
+  console.log('doing busy work');
+  for (let i = 0; i <= 40000000; i++) {} // short delay
+
+  // 6. Annotate our span to capture metadata about our operation
+  span.addAnnotation('invoking doWork');
+  for (let i = 0; i <= 20000000; i++) {} // short delay
+
+  span.end();
+}
+
+main();