Initial Checkin.

Only DHT (AM3202) supported at this time.

README.md

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+# Prometheus Sensors
+
+This is a library that implements a set of sensor types, reading their values
+periodically and exposing them to Prometheus using the `prometheus-metrics`
+library.
+
+## Supported Sensors
+
+### DHT 
+
+This is using `dht` library; each sensor reading takes approximately 32ms (28ms
+if there is no sensor on the GPIO in). They are placed on timers so as to
+minimize pre-emption of the main application. DHT sensors are polled in order
+based on `sensors.dht_gpio` pins and with a period of `sensors.dht_period`
+which defaults to 3 seconds.