Add Prometheus timer.

This timer will periodically upload Prometheus information to the push
gateway. By default, every `sensors.pushgateway_period` (10) seconds.

src/mgos_prometheus_sensors.c

@@ -1,9 +1,17 @@
 #include "mgos_config.h"
+#include "mgos_prometheus_metrics.h"
 #include "mgos_prometheus_sensors.h"
 
 void dht_init();
 
+static void pushgateway_timer(void *user_data) {
+  mgos_prometheus_metrics_push(MGOS_APP, mgos_sys_config_get_device_id());
+  (void) user_data;
+}
+
 bool mgos_prometheus_sensors_init(void) {
   dht_init();
+  if (mgos_sys_config_get_sensors_pushgateway_period()>0)
+    mgos_set_timer(mgos_sys_config_get_sensors_pushgateway_period()*1000, true, pushgateway_timer, NULL);
   return true;
 }