add docs for the metrics

docs/metrics.md

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+# Metrics
+
+This TETRA reception stack comes with a lot of usefull metrics for monitoring the health and troughput of a cell.
+All metrics are available through a prometheus exposer configurable with the command line arguments `--prometheus-address` and `--prometheus-name`.
+
+Following metrics are supported:
+
+| Metric name | Type | Description | Labels |
+|---|---|---|---|
+| `burst_received_count` | Counter | Counters for received bursts | `burst_type`: The type of received burst |
+| `burst_lower_mac_decode_error_count` | Counter | Counters for decoding errors on received bursts in the lower MAC | `burst_type`: The type of received burst |
+| `burst_lower_mac_mismatch_count` | Counter | Counters for mismatched number of bursts in the downlink lower MAC | `mismatch_type`: Any of `Skipped` or `Too many` |
+| `lower_mac_time_gauge` | Gauge | Gauges for the network time | `type`: Any of `Synchronization Burst` or `Prediction` |
+| `upper_mac_total_slot_count` | Counter | Counters for all received slots | `logical_channel`: The logical channel that is contained in the slot. |
+| `upper_mac_slot_error_count` | Counter | Counters for all received slots with errors | `logical_channel`: The logical channel that is contained in the slot. `error_type`: Any of `CRC Error` or `Decode Error`. |
+| `upper_mac_packet_count` | Counter | Counters for all received upper mac packets | `packet_type`: The type of reveiced upper mac packet. |
+| `upper_mac_fragment_count` | Counter | Counters for all received c-plane fragments | `type`: Any of `Continous` or `Stealing Channel`. `counter_type`: Any of `All` or `Reconstuction Error`. |
+| `c_plane_packet_count` | Counter | Counters for all received c-plane packets | `packet_type`: The mac packet type. (Start fragments are seperated) |