Updated readme

Readme.md

@@ -8,28 +8,27 @@ publish the received messages as prometheus metrics. I wrote this exporter to pu
 metrics from small embedded sensors based on the NodeMCU to prometheus. The used arduino scetch can be found in the [dht22tomqtt](https://github.com/hikhvar/dht22tomqtt) repository.
 
 ## Assumptions about Messages and Topics
-This exporter makes some assumptions about the message format and MQTT topics. This exporter assumes that each
-client publish the metrics into a dedicated topic. The last level topic becomes the `sensor` label in prometheus.
-This exporter assume that the message are JSON objects with only float fields. The golang type for the messages is: 
-
-```go
-type MQTTPayload map[string]float64
-```
-
-For example the message
+This exporter makes some assumptions about the MQTT topics. This exporter assumes that each
+client publish the metrics into a dedicated topic. The regular expression ìn the configuration field `mqtt.device_id_regex`
+defines how to extract the device ID from the MQTT topic. This allow an arbitrary place of the device ID in the mqtt topic.
+For example the [tasmota](https://github.com/arendst/Tasmota) firmware pushes the telemetry data to the topics `tele/<deviceid>/SENSOR`.
 
+Let us assume the default configuration from [#ConfigFile]. A sensor publishes the following message
 ```json
-{"temperature":23.20,"humidity":51.60,"heat_index":22.92}
+{"temperature":23.20,"humidity":51.60, "computed": {"heat_index":22.92} }
 ```
 
-published to the MQTT topic `devices/me/livingroom` becomes the following prometheus metrics:
+to the MQTT topic `devices/me/livingroom`. This message becomes the following prometheus metrics:
 
 ```text
-temperature{sensor="livingroom"} 23.2
-heat_index{sensor="livingroom"} 22.92
-humidity{sensor="livingroom"} 51.6
+temperature{sensor="livingroom",topic="devices/me/livingroom"} 23.2
+heat_index{sensor="livingroom",topic="devices/me/livingroom"} 22.92
+humidity{sensor="livingroom",topic="devices/me/livingroom"} 51.6
 ```
 
+### Tasmota
+An example configuration for the tasmota based Gosund SP111 device is given in [examples/gosund_sp111.yaml](examples/gosund_sp111.yaml).
+
 ## Build
 
 To build the exporter run:
@@ -85,18 +84,23 @@ The config file can look like this:
 mqtt:
   # The MQTT broker to connect to
   server: tcp://127.0.0.1:1883
-  # The Topic path to subscripe to. Actually this will become `$topic_path/+`
-  topic_path: v1/devices/me
+  # The Topic path to subscripe to. Be aware that you have to specify the wildcard. 
+  topic_path: v1/devices/me/+
+  # Optional: Regular expression to extract the device ID from the topic path. The default regular expression, assumes
+  # that the last "element" of the topic_path is the device id.
+  # The regular expression must contain a named capture group with the name deviceid
+  # For example the expression for tasamota based sensors is "tele/(?P<deviceid>.*)/.*"
+  device_id_regex: "(.*/)?(?P<deviceid>.*)"
   # The MQTT QoS level
   qos: 0
 cache:
   # Timeout. Each received metric will be presented for this time if no update is send via MQTT
-  timeout: 2min
+  timeout: 24h
 # This is a list of valid metrics. Only metrics listed here will be exported
 metrics:
     # The name of the metric in prometheus
   - prom_name: temperature_celsius
-    # The name of the metric in a MQTT JSON message
+    # The name of the metric in a MQTT JSON message. This can be an arbitrary gojsonq path.
     mqtt_name: temperature
     # The prometheus help text for this metric
     help: DHT22 temperature reading
@@ -118,8 +122,8 @@ metrics:
       sensor_type: dht22
     # The name of the metric in prometheus
   - prom_name: heat_index
-    # The name of the metric in a MQTT JSON message
-    mqtt_name: heat_index
+    # The name of the metric in a MQTT JSON message. Here a nested field.
+    mqtt_name: computed.heat_index
     # The prometheus help text for this metric
     help: DHT22 heatIndex calculation
     # The prometheus type for this metric. Valid values are: "gauge" and "counter"