[Jaeger-Exporter] host default env var

packages/opentelemetry-exporter-jaeger/README.md

@@ -1,4 +1,5 @@
 # OpenTelemetry Jaeger Trace Exporter
+
 [![Gitter chat][gitter-image]][gitter-url]
 [![NPM Published Version][npm-img]][npm-url]
 [![dependencies][dependencies-image]][dependencies-url]
@@ -54,6 +55,10 @@ npm install --save @opentelemetry/exporter-jaeger
 
 Install the exporter on your application and pass the options, it must contain a service name.
 
+Furthermore, the `host` option (which defaults to `localhost`), can instead be set by the
+`JAEGER_AGENT_HOST` environment variable to reduce in-code config. If both are
+set, the value set by the option in code is authoritative.
+
 ```js
 import { JaegerExporter } from '@opentelemetry/exporter-jaeger';
 
@@ -78,8 +83,8 @@ You can use built-in `SimpleSpanProcessor` or `BatchSpanProcessor` or write your
 - [SimpleSpanProcessor](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-tracing.md#simple-processor): The implementation of `SpanProcessor` that passes ended span directly to the configured `SpanExporter`.
 - [BatchSpanProcessor](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-tracing.md#batching-processor): The implementation of the `SpanProcessor` that batches ended spans and pushes them to the configured `SpanExporter`. It is recommended to use this `SpanProcessor` for better performance and optimization.
 
-
 ## Useful links
+
 - To know more about Jaeger, visit: https://www.jaegertracing.io/docs/latest/getting-started/
 - For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
 - For more about OpenTelemetry JavaScript: <https://github.com/open-telemetry/opentelemetry-js>

packages/opentelemetry-exporter-jaeger/src/jaeger.ts

@@ -37,6 +37,8 @@ export class JaegerExporter implements SpanExporter {
     this._onShutdownFlushTimeout =
       typeof config.flushTimeout === 'number' ? config.flushTimeout : 2000;
 
+    config.host = config.host || process.env.JAEGER_AGENT_HOST;
+
     this._sender = new jaegerTypes.UDPSender(config);
     if (this._sender._client instanceof Socket) {
       // unref socket to prevent it from keeping the process running

packages/opentelemetry-exporter-jaeger/test/jaeger.test.ts

@@ -26,6 +26,10 @@ import { Resource } from '@opentelemetry/resources';
 
 describe('JaegerExporter', () => {
   describe('constructor', () => {
+    afterEach(() => {
+      delete process.env.JAEGER_AGENT_HOST;
+    });
+
     it('should construct an exporter', () => {
       const exporter = new JaegerExporter({ serviceName: 'opentelemetry' });
       assert.ok(typeof exporter.export === 'function');
@@ -38,7 +42,7 @@ describe('JaegerExporter', () => {
     it('should construct an exporter with host, port, logger and tags', () => {
       const exporter = new JaegerExporter({
         serviceName: 'opentelemetry',
-        host: 'localhost',
+        host: 'remotehost',
         port: 8080,
         logger: new NoopLogger(),
         tags: [{ key: 'opentelemetry-exporter-jaeger', value: '0.1.0' }],
@@ -47,13 +51,38 @@ describe('JaegerExporter', () => {
       assert.ok(typeof exporter.shutdown === 'function');
 
       const process: ThriftProcess = exporter['_sender']._process;
+      assert.strictEqual(exporter['_sender']._host, 'remotehost');
       assert.strictEqual(process.serviceName, 'opentelemetry');
       assert.strictEqual(process.tags.length, 1);
       assert.strictEqual(process.tags[0].key, 'opentelemetry-exporter-jaeger');
       assert.strictEqual(process.tags[0].vType, 'STRING');
       assert.strictEqual(process.tags[0].vStr, '0.1.0');
     });
 
+    it('should default to localhost if no host is configured', () => {
+      const exporter = new JaegerExporter({
+        serviceName: 'opentelemetry',
+      });
+      assert.strictEqual(exporter['_sender']._host, 'localhost');
+    });
+
+    it('should respect jaeger host env variable', () => {
+      process.env.JAEGER_AGENT_HOST = 'env-set-host';
+      const exporter = new JaegerExporter({
+        serviceName: 'test-service',
+      });
+      assert.strictEqual(exporter['_sender']._host, 'env-set-host');
+    });
+
+    it('should prioritize host option over env variable', () => {
+      process.env.JAEGER_AGENT_HOST = 'env-set-host';
+      const exporter = new JaegerExporter({
+        serviceName: 'test-service',
+        host: 'option-set-host',
+      });
+      assert.strictEqual(exporter['_sender']._host, 'option-set-host');
+    });
+
     it('should construct an exporter with flushTimeout', () => {
       const exporter = new JaegerExporter({
         serviceName: 'opentelemetry',