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.

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)