Small embedded devices tend have a serial port of some sort and some ad-hoc protocol or mechanism to trigger metric output. A prime example of this that comes to mind is the P1 port on some of the European electricity meters. This user-accessible port outputs some metrics every so often. Specification for the curious is here: https://www.netbeheernederland.nl/_upload/Files/Slimme_meter...

ARM chips intended for embedded uses also have a single wire output peripheral for tracing and similar purposes that could be used for this.