Add uptime metric to prometheus exporter

I'm following the naming practices:
https://prometheus.io/docs/practices/naming/
- an application prefix (tasmota_)
- a unit suffix in plural form

Tested building and flashing: looks like

```
tasmota_uptime_seconds 78
```

tasmota/xsns_91_prometheus.ino

@@ -37,6 +37,8 @@ void HandleMetrics(void)
 
   char parameter[FLOATSZ];
 
+  WSContentSend_P(PSTR("# TYPE tasmota_uptime_seconds gauge\ntasmota_uptime_seconds %d\n"), uptime);
+
   if (!isnan(global_temperature)) {
     dtostrfd(global_temperature, Settings.flag2.temperature_resolution, parameter);
     WSContentSend_P(PSTR("# TYPE global_temperature gauge\nglobal_temperature %s\n"), parameter);