[express] Add option to record HTTP headers.

Author: alloy

docs/API.md

@@ -136,13 +136,15 @@ Each integration also has its own list of default tags. These tags get automatic
 | http.url         | The complete URL of the request.                          |
 | http.method      | The HTTP method of the request.                           |
 | http.status_code | The HTTP status code of the response.                     |
+| http.headers.*   | A recorded HTTP header.                                   |
 
 <h5 id="express-config">Configuration Options</h5>
 
 | Option           | Default                   | Description                            |
 |------------------|---------------------------|----------------------------------------|
 | service          | *Service name of the app* | The service name for this integration. |
 | validateStatus   | `code => code < 500`      | Callback function to determine if there was an error. It should take a status code as its only parameter and return `true` for success or `false` for errors. |
+| recordHeaders    | `undefined`               | An array of headers to include in the span metadata. |
 
 <h3 id="graphql">graphql</h3>
 

src/plugins/express.js

@@ -46,6 +46,16 @@ function createWrapMethod (tracer, config) {
         span.setTag(Tags.ERROR, true)
       }
 
+      if (config.recordHeaders) {
+        config.recordHeaders.forEach(key => {
+          key = key.toLowerCase()
+          const value = req.headers[key]
+          if (value) {
+            span.setTag(`http.headers.${key}`, value)
+          }
+        })
+      }
+
       span.finish()
 
       req._datadog.scope && req._datadog.scope.close()

test/plugins/express.spec.js

@@ -546,7 +546,8 @@ describe('Plugin', () => {
         beforeEach(() => {
           config = {
             service: 'custom',
-            validateStatus: code => code < 400
+            validateStatus: code => code < 400,
+            recordHeaders: ['User-Agent']
           }
 
           return agent.load(plugin, 'express', config)
@@ -602,6 +603,31 @@ describe('Plugin', () => {
             })
           })
         })
+
+        it('should include specified headers in metadata', done => {
+          const app = express()
+
+          app.get('/user', (req, res) => {
+            res.status(200).send()
+          })
+
+          getPort().then(port => {
+            agent
+              .use(traces => {
+                expect(traces[0][0].meta).to.have.property('http.headers.user-agent', 'test')
+              })
+              .then(done)
+              .catch(done)
+
+            appListener = app.listen(port, 'localhost', () => {
+              axios
+                .get(`http://localhost:${port}/user`, {
+                  headers: { 'User-Agent': 'test' }
+                })
+                .catch(done)
+            })
+          })
+        })
       })
     })
   })