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_helper/Helper.cpp

@@ -1206,3 +1206,13 @@ bool isInString(std::string &s, std::string const &toFind)
 
 	return true;
 }
+
+// from https://stackoverflow.com/a/14678800
+void replaceAll(std::string& s, const std::string& toReplace, const std::string& replaceWith)
+{
+    size_t pos = 0;
+    while ((pos = s.find(toReplace, pos)) != std::string::npos) {
+         s.replace(pos, toReplace.length(), replaceWith);
+         pos += replaceWith.length();
+    }
+}