Experimental Prometheus support (#1943)

* Experimental Prometheus support
Refactors text response to accept responseType
Generates a plaintext output in prometheus format using standard units and factors
Only called if and when endpoint (/metrics) is accessed
Exceptions are reported in the text output
This tries to minimize changes and uses no extra dependencies

* Changed strings to interpolation and raw literals to improve readability

* Fixed a typo

* Adding the Async HttpServer from Master

* Added back the http headers to SendResponseAsync

* Adding no-cache to sensor and prometheus