web: prometheus metrics support (#2332)

- (experimental) provide generic way to read magnitude values
- expose /api/metrics with values formatted specifically for prometheus, with relay and sensor data
- small tweaks to sensor init

Example config:
```
scrape_configs:
  - job_name: 'espurna'
    metrics_path: '/api/metrics'
    params:
      apikey: ['apikeyapikey']
    static_configs:
      - targets: ['espurna-blabla.lan:80']
```
Where ESPurna side has
```
apiKey => "apikeyapikey"
apiEnabled => "1"
```

code/espurna/web.cpp

@@ -469,10 +469,11 @@ void _onRequest(AsyncWebServerRequest *request){
 
     if (!_onAPModeRequest(request)) return;
 
-    // Send request to subscribers
-    for (unsigned char i = 0; i < _web_request_callbacks.size(); i++) {
-        bool response = (_web_request_callbacks[i])(request);
-        if (response) return;
+    // Send request to subscribers, break when request is 'handled' by the callback
+    for (auto& callback : _web_request_callbacks) {
+        if (callback(request)) {
+            return;
+        }
     }
 
     // No subscriber handled the request, return a 404 with implicit "Connection: close"