Add Read Cache and Stats

- baro.cache_ttl_ms avoids multiple reads
- by default, cache is off (cache_ttl_ms=0)
- stats are updated (similar to how prometheus-sensors does this)

src/mgos_barometer_internal.h

@@ -32,6 +32,7 @@ typedef bool (*mgos_barometer_mag_read_fn)(struct mgos_barometer *dev);
 struct mgos_barometer {
   struct mgos_i2c *      i2c;
   uint8_t                i2caddr;
+  uint16_t          cache_ttl_ms;
   enum mgos_barometer_type type;
   bool has_thermometer;
   bool has_barometer;
@@ -45,6 +46,8 @@ struct mgos_barometer {
 
   float pressure; // in Pascals
   float temperature; // in Celcius
+
+  struct mgos_barometer_stats stats;
 };
 
 #ifdef __cplusplus