add documentation prometheus integration (#55)

docs/prometheus-openmetrics.md

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+# Prometheus/OpenMetrics
+
+A set of metrics is exported via the `/metrics` path on the device. Besides the current value, a set of device properties are exported. Multiple sequences (aka *numbers*) are supported via a label. The metrics are provided in text wire format.
+
+Example:
+
+```bash
+$ curl http://192.168.178.23/metrics
+# HELP ai_on_the_edge_device_flow_value current value of meter readout
+# TYPE ai_on_the_edge_device_flow_value gauge
+ai_on_the_edge_device_flow_value{sequence="main"} 240.7064
+# HELP ai_on_the_edge_device_cpu_temperature_celsius current cpu temperature in celsius
+# TYPE ai_on_the_edge_device_cpu_temperature_celsius gauge
+ai_on_the_edge_device_cpu_temperature_celsius 41
+# HELP ai_on_the_edge_device_rssi_dbm current WiFi signal strength in dBm
+# TYPE ai_on_the_edge_device_rssi_dbm gauge
+ai_on_the_edge_device_rssi_dbm -67
+# HELP ai_on_the_edge_device_memory_heap_free_bytes available heap memory
+# TYPE ai_on_the_edge_device_memory_heap_free_bytes gauge
+ai_on_the_edge_device_memory_heap_free_bytes 716303
+# HELP ai_on_the_edge_device_uptime_seconds device uptime in seconds
+# TYPE ai_on_the_edge_device_uptime_seconds gauge
+ai_on_the_edge_device_uptime_seconds 214267
+# HELP ai_on_the_edge_device_rounds_total data aquisition rounds since device startup
+# TYPE ai_on_the_edge_device_rounds_total counter
+ai_on_the_edge_device_rounds_total 239
+```
+
+## Prometheus Scrape Config
+
+The following scrape config (add to `prometheus.yml`) can be used as an example to ingest available metrics with prometheus:
+
+```
+scrape_configs:
+  - job_name: watermeter
+    scrape_interval: 300s
+    metrics_path: /metrics
+    static_configs:
+      - targets: ['192.168.178.23']
+```
+
+## References
+
+- [OpenMetrics](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md)