Log error on appsignal.tracer function call

We notice that some people, after upgrading to version 3 of our package,
run into crashes with undescriptive error messages.
This happens when `appsignal.tracer()` is called.

To help with the upgrade process, log a message with a link to the
upgrade guide.

.changesets/log-error-for-appsignal-tracer-function-calls-.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "change"
+---
+
+Log an error for appsignal.tracer function calls. Inform the user to update instrumentation code to be compatible with the latest version of our package.

src/client.ts

@@ -386,4 +386,10 @@ export class Client {
   private storeInGlobal(): void {
     global.__APPSIGNAL__ = this
   }
+
+  tracer(): void {
+    console.error(
+      "The `appsignal.tracer()` function was called, but it has been removed in AppSignal for Node.js package version 3.x. Please read our migration guide to upgrade to this new version of our package: https://docs.appsignal.com/nodejs/3.x/migration-guide.html. It is also possible to downgrade to version 2.x, after which this code will work again."
+    )
+  }
 }