Support field customization

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 */
+  customize: (fields: 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.customize && opts.customize(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({
+      customize(fields, req, res) {
+        fields.labels ??= {};
+        fields.labels.custom = 'customValue';
+      }
+    }),
+    { stream }
+  )
+
+  makeExpressServerAndRequest(logger, '/', {}, null, function (err) {
+    t.error(err)
+    t.end()
+  })
+})