add Prometheus/OpenMetrics exporter (#3081)

* add prometheus endpoint

* refine metrics implementation

* move metrics generator to ClassFlowControll

* add more metrics
align prefix

* add more metrics
clean up

* refine documentation

* revert dependencies change

* sanitize labels

* create separate module for openmetrics

* move openmetrics to separate folder

* clean up

* add basic unit-tests

* work with const numbers
add replaceAll for string replacement
avoid opening std namespace
adapt unit-tests

* Update code/main/server_main.cpp

---------

Co-authored-by: CaCO3 <caco3@ruinelli.ch>

code/components/jomjol_flowcontroll/MainFlowControl.h

@@ -9,6 +9,7 @@
 #include <esp_http_server.h>
 #include "CImageBasis.h"
 #include "ClassFlowControll.h"
+#include "openmetrics.h"
 
 typedef struct
 {