Some finishing touches.

- Print *_timer_cb consistently (add units "C" and "%" at .2f precision)
- Guard HTU21DF and SI7021 from co-existing (both devices are on the same I2C address)
- Differentiate between BME280 and BMP280 in metrics
- Update README with timing information
- Link in all drivers

src/mgos_prometheus_sensors.c

@@ -5,6 +5,10 @@
 void dht_drv_init();
 void veml6075_drv_init();
 void bme280_drv_init();
+void sht31_drv_init();
+void si7021_drv_init();
+void htu21df_drv_init();
+void mcp9808_drv_init();
 
 static void pushgateway_timer(void *user_data) {
   mgos_prometheus_metrics_push(MGOS_APP, mgos_sys_config_get_device_id());
@@ -15,6 +19,11 @@ bool mgos_prometheus_sensors_init(void) {
   dht_drv_init();
   veml6075_drv_init();
   bme280_drv_init();
+  si7021_drv_init();
+  sht31_drv_init();
+  htu21df_drv_init();
+  mcp9808_drv_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;