docs: add network io procfs system provider (#114)

Author: ribalba

content/en/docs/measuring/metric-providers/network-io-procfs-system.md

@@ -0,0 +1,62 @@
+---
+title: "Network IO - procfs - system"
+description: "Documentation for NetworkIoProcfsSystemProvider of the Green Metrics Tool"
+date: 2025-07-22T17:50:00+00:00
+weight: 201
+---
+
+### What it does
+
+It reads the cumulative amount of bytes received and transmitted on each
+network interface of the host by parsing `/proc/net/dev`. This allows
+monitoring system-wide network usage independent of containers.
+
+### Classname
+
+- `NetworkIoProcfsSystemProvider`
+
+### Metric Name
+
+- `network_io_procfs_system`
+
+### Input Parameters
+
+- args
+  - `-i`: interval in milliseconds
+
+By default the measurement interval is 100 ms.
+
+```bash
+./metric-provider-binary -i 100
+```
+
+### Output
+
+This metric provider prints to Stdout a continuous stream of data. The format of
+ the data is as follows:
+
+`TIMESTAMP RECEIVED_BYTES TRANSMITTED_BYTES INTERFACE`
+
+Where:
+- `TIMESTAMP`: Unix timestamp, in microseconds
+- `RECEIVED_BYTES`: Total bytes received by the interface since system boot
+- `TRANSMITTED_BYTES`: Total bytes transmitted by the interface since system boot
+- `INTERFACE`: Name of the network interface (for example, eth0)
+
+Any errors are printed to Stderr.
+
+### How it works
+
+The provider reads the counters from `/proc/net/dev` for every network
+interface. The values are sorted by interface and timestamp. Optionally, virtual
+interfaces are filtered out so only physical network adapters are included.
+
+For each interface, the difference between consecutive measurements is computed
+internally to obtain the bytes transmitted during the sampling interval. The
+sum of received and transmitted bytes forms the final value reported for the
+interval.
+
+System-wide network traffic is not attributed to specific containers. When
+analyzing energy consumption based on these values, you may combine them with the
+[Network Carbon Intensity →]({{< relref "../carbon/network-carbon-intensity" >}})
+page for further guidance.