Avoid probes interval blocking shutdown

The probes instance starts an interval timer on initialisation.
By default, timers cause the Node.js engine to await their
completion on shutdown -- which, this being an interval timer, is
never actually completed. (See #418)

We fixed this in the past by implementing `Appsignal.stop`, which
clears the interval. But a preferable solution is to mark the
probes' interval as `.unref()`, telling the Node.js engine that it
does not need to be awaited.

.changesets/do-not-block-node-js-shutdown.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: change
+---
+
+Do not block Node.js shutdown. It is no longer necessary to call `Appsignal.stop` for the Node.js engine to allow itself to shut down. It should still be called and awaited in production scenarios and at the end of scripts, as it ensures that scheduled check-ins are delivered.

src/probes/index.ts

@@ -71,7 +71,7 @@ class BaseProbeRunner extends EventEmitter implements ProbeRunner {
   public register(name: string, fn: () => void): void {
     this.#timers.set(
       name,
-      setInterval(() => this.emit(name), 60 * 1000)
+      setInterval(() => this.emit(name), 60 * 1000).unref()
     )
 
     this.removeAllListeners(name)