Support field customization

docs/morgan.asciidoc

@@ -204,6 +204,35 @@ Integration with Elastic APM can be explicitly disabled via the
 app.use(morgan(ecsFormat({ apmIntegration: false })));
 ----
 
+[float]
+[[morgan-logHook]]
+=== Customizing Log Records
+
+To add custom fields to log records or remove sensitive data, you can use the `logHook` option. This option is a function that takes the formatted log record and returns a new log record. Here's an example that adds a label to the log record and removes the `authorization` and `cookie` headers:
+
+[source,js]
+----
+app.use(morgan(ecsFormat({
+  logHook: ({ record, req, res }) => {
+    // Omit sensitive headers
+    const { authorization, cookie, ...headers } = record.http.request.headers;
+    return {
+      ...record,
+      labels: {
+        ...record.labels,
+        customLabel: res.locals.someValue
+      },
+      http: {
+        ...record.http,
+        request: {
+          ...record.http.request,
+          headers
+        }
+      }
+    };
+  },
+})));
+----
 
 [float]
 [[morgan-ref]]
@@ -222,5 +251,6 @@ app.use(morgan(ecsFormat({ apmIntegration: false })));
 ** `serviceEnvironment` +{type-string}+ A "service.environment" value. If specified this overrides any value from an active APM agent.
 ** `serviceNodeName` +{type-string}+ A "service.node.name" value. If specified this overrides any value from an active APM agent.
 ** `eventDataset` +{type-string}+ A "event.dataset" value. If specified this overrides the default of using `${serviceVersion}`.
+** `logHook` +{type-function}+ A function that takes the formatted log record and returns a new log record. This can be used to add custom fields to the log record or remove sensitive data. It is passed an object with three properties: `record`, `req`, and `res`.
 
 Create a formatter for morgan that emits in ECS Logging format.

packages/ecs-morgan-format/index.d.ts

@@ -1,3 +1,4 @@
+import type { IncomingMessage, ServerResponse } from 'http';
 import type { FormatFn } from "morgan";
 
 interface Config {
@@ -42,6 +43,8 @@ interface Config {
   /** Specify "event.dataset" field. Defaults `${serviceName}`. */
   eventDataset?: string;
 
+  /** Callback for custom modification of the fields */
+  logHook: (event: { record: any, req: IncomingMessage, res: ServerResponse }) => void;
 }
 
 declare function ecsFormat(config?: Config): FormatFn;

packages/ecs-morgan-format/index.js

@@ -175,6 +175,8 @@ function ecsFormat (opts) {
     formatHttpRequest(ecsFields, req)
     formatHttpResponse(ecsFields, res)
 
+    opts.logHook && opts.logHook({ record: ecsFields, req, res })
+
     return stringify(ecsFields)
   }
 }

packages/ecs-morgan-format/test/basic.test.js

@@ -235,3 +235,26 @@ test('can configure correlation fields', t => {
     t.end()
   })
 })
+
+test('can provide custom fields', t => {
+  t.plan(2)
+
+  const stream = split().on('data', line => {
+    const rec = JSON.parse(line)
+    t.equal(rec.labels.custom, 'customValue')
+  })
+  const logger = morgan(
+    ecsFormat({
+      logHook ({ record, req, res }) {
+        record.labels = record.labels || {}
+        record.labels.custom = 'customValue'
+      }
+    }),
+    { stream }
+  )
+
+  makeExpressServerAndRequest(logger, '/', {}, null, function (err) {
+    t.error(err)
+    t.end()
+  })
+})