Update express example to include route filtering (open-telemetry#652)

Fixes open-telemetry#651

Author: hermanbanken

examples/express/server.js

@@ -32,6 +32,7 @@ const authMiddleware = (req, res, next) => {
 };
 
 app.use(express.json());
+app.get('/health', (req, res) => res.status(200).send("HEALTHY")); // endpoint that is called by framework/cluster
 app.get('/run_test', async (req, res) => {
   // Calls another endpoint of the same API, somewhat mimicing an external API call
   const createdCat = await axios.post(`http://localhost:${PORT}/cats`, {

examples/express/tracer.js

@@ -6,13 +6,14 @@ const opentelemetry = require('@opentelemetry/api');
 const { diag, DiagConsoleLogger, DiagLogLevel } = opentelemetry;
 diag.setLogger(new DiagConsoleLogger(), DiagLogLevel.INFO);
 
+const { AlwaysOnSampler } = require('@opentelemetry/core');
 const { registerInstrumentations } = require('@opentelemetry/instrumentation');
 const { NodeTracerProvider } = require('@opentelemetry/sdk-trace-node');
 const { SimpleSpanProcessor } = require('@opentelemetry/sdk-trace-base');
 const { JaegerExporter } = require('@opentelemetry/exporter-jaeger');
 const { ZipkinExporter } = require('@opentelemetry/exporter-zipkin');
 const { Resource } = require('@opentelemetry/resources');
-const { SemanticResourceAttributes: ResourceAttributesSC } = require('@opentelemetry/semantic-conventions');
+const { SemanticAttributes, SemanticResourceAttributes: ResourceAttributesSC } = require('@opentelemetry/semantic-conventions');
 
 const Exporter = (process.env.EXPORTER || '')
   .toLowerCase().startsWith('z') ? ZipkinExporter : JaegerExporter;
@@ -24,6 +25,7 @@ module.exports = (serviceName) => {
     resource: new Resource({
       [ResourceAttributesSC.SERVICE_NAME]: serviceName,
     }),
+    sampler: filterSampler(ignoreHealthCheck, new AlwaysOnSampler()),
   });
   registerInstrumentations({
     tracerProvider: provider,
@@ -45,3 +47,21 @@ module.exports = (serviceName) => {
 
   return opentelemetry.trace.getTracer('express-example');
 };
+
+function filterSampler(filterFn, parent) {
+  return {
+    shouldSample(ctx, tid, spanName, spanKind, attr, links) {
+      if (!filterFn(spanName, spanKind, attr)) {
+        return { decision: opentelemetry.SamplingDecision.NOT_RECORD };
+      }
+      return parent.shouldSample(ctx, tid, name, kind, attr, links);
+    },
+    toString() {
+      return `FilterSampler(${parent.toString()})`;
+    }
+  }
+}
+
+function ignoreHealthCheck(spanName, spanKind, attributes) {
+  return spanKind !== opentelemetry.SpanKind.SERVER || attributes[SemanticAttributes.HTTP_ROUTE] !== "/health";
+}