docs(grpc): add grpc example (#326)

* docs(grpc): add grpc example

* docs(readme): add zipkin ui image

* fix: typos, add local images, refactor

* fix: typo

examples/grpc/README.md

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+# Overview
+
+OpenTelemetry gRPC Instrumentation allows the user to automatically collect trace data and export them to the backend of choice (we can use Zipkin or Jaeger for this example), to give observability to distributed systems.
+
+## Installation
+
+```sh
+$ # from this directory
+$ npm install
+```
+
+Setup [Zipkin Tracing](https://zipkin.io/pages/quickstart.html)
+or
+Setup [Jaeger Tracing](https://www.jaegertracing.io/docs/latest/getting-started/#all-in-one)
+
+## Run the Application
+
+### Zipkin
+
+ - Run the server
+
+   ```sh
+   $ # from this directory
+   $ npm run zipkin:server
+   ```
+
+ - Run the client
+
+   ```sh
+   $ # from this directory
+   $ npm run zipkin:client
+   ```
+
+#### Zipkin UI
+`zipkin:server` script should output the `traceid` in the terminal (e.g `traceid: 4815c3d576d930189725f1f1d1bdfcc6`).
+Go to Zipkin with your browser [http://localhost:9411/zipkin/traces/(your-trace-id)]() (e.g http://localhost:9411/zipkin/traces/4815c3d576d930189725f1f1d1bdfcc6)
+
+<p align="center"><img src="./images/zipkin.png"/></p>
+
+### Jaeger
+
+ - Run the server
+
+   ```sh
+   $ # from this directory
+   $ npm run jaeger:server
+   ```
+
+ - Run the client
+
+   ```sh
+   $ # from this directory
+   $ npm run jaeger:client
+   ```
+#### Jaeger UI
+
+`jaeger:server` script should output the `traceid` in the terminal (e.g `traceid: 4815c3d576d930189725f1f1d1bdfcc6`).
+Go to Jaeger with your browser [http://localhost:50051/trace/(your-trace-id)]() (e.g http://localhost:50051/trace/4815c3d576d930189725f1f1d1bdfcc6)
+
+<p align="center"><img src="./images/jaeger.png"/></p>
+
+## Useful links
+- For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
+- For more information on OpenTelemetry for Node.js, visit: <https://github.com/open-telemetry/opentelemetry-js/tree/master/packages/opentelemetry-node-sdk>
+
+## LICENSE
+
+Apache License 2.0

examples/grpc/client.js

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+'use strict';
+
+const opentelemetry = require('@opentelemetry/core');
+const config = require('./setup');
+
+/**
+ * The trace instance needs to be initialized first, if you want to enable
+ * automatic tracing for built-in plugins (gRPC in this case).
+ */
+config.setupTracerAndExporters('grpc-client-service');
+
+const grpc = require('grpc');
+
+const messages = require('./helloworld_pb');
+const services = require('./helloworld_grpc_pb');
+const PORT = 50051;
+const tracer = opentelemetry.getTracer();
+
+/** A function which makes requests and handles response. */
+function main() {
+  // span corresponds to outgoing requests. Here, we have manually created
+  // the span, which is created to track work that happens outside of the
+  // request lifecycle entirely.
+  const span = tracer.startSpan('client.js:main()');
+  tracer.withSpan(span, () => {
+    console.log('Client traceId ', span.context().traceId);
+    const client = new services.GreeterClient(
+      `localhost:${PORT}`,
+      grpc.credentials.createInsecure()
+    );
+    const request = new messages.HelloRequest();
+    let user;
+    if (process.argv.length >= 3) {
+      user = process.argv[2];
+    } else {
+      user = 'world';
+    }
+    request.setName(user);
+    client.sayHello(request, function(err, response) {
+      span.end();
+      if (err) throw err;
+      console.log('Greeting:', response.getMessage());
+    });
+  });
+}
+
+main();

examples/grpc/helloworld_grpc_pb.js

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+// GENERATED CODE -- DO NOT EDIT!
+
+// Original file comments:
+// Copyright 2015 gRPC 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.
+//
+'use strict';
+var grpc = require('grpc');
+var helloworld_pb = require('./helloworld_pb.js');
+
+function serialize_HelloReply(arg) {
+  if (!(arg instanceof helloworld_pb.HelloReply)) {
+    throw new Error('Expected argument of type HelloReply');
+  }
+  return Buffer.from(arg.serializeBinary());
+}
+
+function deserialize_HelloReply(buffer_arg) {
+  return helloworld_pb.HelloReply.deserializeBinary(new Uint8Array(buffer_arg));
+}
+
+function serialize_HelloRequest(arg) {
+  if (!(arg instanceof helloworld_pb.HelloRequest)) {
+    throw new Error('Expected argument of type HelloRequest');
+  }
+  return Buffer.from(arg.serializeBinary());
+}
+
+function deserialize_HelloRequest(buffer_arg) {
+  return helloworld_pb.HelloRequest.deserializeBinary(
+    new Uint8Array(buffer_arg)
+  );
+}
+
+// The greeting service definition.
+var GreeterService = (exports.GreeterService = {
+  // Sends a greeting
+  sayHello: {
+    path: '/helloworld.Greeter/SayHello',
+    requestStream: false,
+    responseStream: false,
+    requestType: helloworld_pb.HelloRequest,
+    responseType: helloworld_pb.HelloReply,
+    requestSerialize: serialize_HelloRequest,
+    requestDeserialize: deserialize_HelloRequest,
+    responseSerialize: serialize_HelloReply,
+    responseDeserialize: deserialize_HelloReply
+  }
+});
+
+exports.GreeterClient = grpc.makeGenericClientConstructor(GreeterService);