Add DHT temp/humidity getters.

+float mgos_prometheus_sensors_dht_get_temp(uint8_t idx);
+float mgos_prometheus_sensors_dht_get_humidity(uint8_t idx);

They return NAN if the index is invalid, or the sensor couldn't be
found. Otherwise, they return the last measured temperature or humidity
values as floats.

include/mgos_prometheus_sensors.h

@@ -4,4 +4,7 @@
 #include "mgos.h"
 #include "mgos_gpio.h"
 
+float mgos_prometheus_sensors_dht_get_temp(uint8_t idx);
+float mgos_prometheus_sensors_dht_get_humidity(uint8_t idx);
+
 #endif // __MGOS_PROMETHEUS_SENSORS_H