Prometheus server is disabled

[thorrak]

Due to issues with AsyncTCP/AsyncWebServer, I had to drop everything Async in order to build a stable web interface. Unfortunately, the Prometheus server was built around Async, so I had to disable it in order to get v15 built.

I'm pretty sure reenabling it will be straightforward (make the same changes I made to convert from Async to synchronous HTTP, conditionally initialize the server based on an extendedSettings toggle) but given the bugs in v14/the move away from the ESP8266, I want to get v15 published sooner rather than later, and therefore will be deferring this work to v16.