Add period char replacement for prometheus

This was a problem for me when trying to get output from an SDS011 Nova
PM sensor. When parsed by the pometheus code it returns labels / metrics
like:

```# TYPE tasmota_sensors_pm2.5_ gauge
tasmota_sensors_pm2.5_{sensor="sds0x1"} 2.2```

 The error from prometheus was:
 ```"append failed" err="invalid metric type \\"5_ gauge\\"```

tasmota/xsns_75_prometheus.ino

@@ -57,6 +57,7 @@ String FormatMetricName(const char *metric) {  // cleanup spaces and uppercases
   String formatted = metric;
   formatted.toLowerCase();
   formatted.replace(" ", "_");
+  formatted.replace(".", "_");
   return formatted;
 }