fix(undici): copy with [email protected] bug that removed req.addHeader (elastic#3963)

Author: trentm

CHANGELOG.asciidoc

@@ -47,6 +47,9 @@ See the <<upgrade-to-v4>> guide.
 * Fix path resolution for requests that contain invalid characters in its
   host header. ({pull}3923[#3923])
 * Fix span names for `getMore` command of mongodb. ({pull}3919[#3919])
+* Fix undici instrumentation to cope with a bug in [email protected] where
+  `request.addHeader()` was accidentally removed. (It was re-added in
+  [email protected].)
 
 [float]
 ===== Chores

lib/instrumentation/modules/undici.js

@@ -133,7 +133,12 @@ function instrumentUndici(agent) {
       propSpan.propagateTraceContextHeaders(
         request,
         function (req, name, value) {
-          req.addHeader(name, value);
+          if (typeof request.addHeader === 'function') {
+            req.addHeader(name, value);
+          } else if (Array.isArray(request.headers)) {
+            // [email protected] accidentally, briefly removed `request.addHeader()`.
+            req.headers.push(name, value);
+          }
         },
       );
     }