input: network_io_metrics: general documentation improvements (#2285)

- Add "Metrics reported" section with table of output metrics
- Add Linux platform availability note
- Add hyperlink to Node Exporter metrics plugin
- Add verbose and test_at_init options to YAML and conf examples
- Standardize config parameter names to lowercase
- Sort configuration parameters alphabetically
- Fix grammar: "returns something the following" -> "returns output similar to the following"
- Improve consistency with other input plugin docs

Fixes #2284.

Signed-off-by: Eric D. Schabell <eric@schabell.org>

Author: eschabell

pipeline/inputs/network-io-metrics.md

@@ -1,25 +1,38 @@
 # Network I/O metrics
 
-The _Network I/O metrics_ (`netif`) input plugin gathers network traffic information of the running system at regular intervals, and reports them.
+The _Network I/O metrics_ (`netif`) input plugin gathers network traffic information of the running system at regular intervals, and reports them. This plugin is available only for Linux.
 
-The Network I/O metrics plugin creates metrics that are log-based, such as JSON payload. For Prometheus-based metrics, see the Node Exporter metrics input plugin.
+The Network I/O metrics plugin creates metrics that are log-based, such as JSON payload. For Prometheus-based metrics, see the [Node Exporter metrics](node-exporter-metrics.md) input plugin.
+
+## Metrics reported
+
+The following table describes the metrics generated by the plugin. Metric names are prefixed with the interface name (for example, `eth0`):
+
+| Key                      | Description                                    |
+|:-------------------------|:-----------------------------------------------|
+| `{interface}.rx.bytes`   | Number of bytes received on the interface.     |
+| `{interface}.rx.packets` | Number of packets received on the interface.   |
+| `{interface}.rx.errors`  | Number of receive errors on the interface.     |
+| `{interface}.tx.bytes`   | Number of bytes transmitted on the interface.  |
+| `{interface}.tx.packets` | Number of packets transmitted on the interface.|
+| `{interface}.tx.errors`  | Number of transmit errors on the interface.    |
 
 ## Configuration parameters
 
 The plugin supports the following configuration parameters:
 
 | Key             | Description                                                                                             | Default |
 |:----------------|:--------------------------------------------------------------------------------------------------------|:--------|
-| `Interface`     | Specify the network interface to monitor. For example, `eth0`.                                          | _none_  |
-| `Interval_Sec`  | Polling interval (seconds).                                                                             | `1`     |
-| `Interval_NSec` | Polling interval (nanosecond).                                                                          | `0`     |
-| `Verbose`       | If true, gather metrics precisely.                                                                      | `false` |
-| `Test_At_Init`  | If true, testing if the network interface is valid at initialization.                                   | `false` |
-| `Threaded`      | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs). | `false` |
+| `interface`     | Specify the network interface to monitor. For example, `eth0`.                                          | _none_  |
+| `interval_nsec` | Polling interval in nanoseconds.                                                                        | `0`     |
+| `interval_sec`  | Polling interval in seconds.                                                                            | `1`     |
+| `test_at_init`  | If true, test if the network interface is valid at initialization.                                      | `false` |
+| `threaded`      | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs). | `false` |
+| `verbose`       | If true, gather metrics precisely.                                                                      | `false` |
 
 ## Get started
 
-To monitor network traffic from your system, you can run the plugin from the command line or through the configuration file:
+To monitor network traffic from your system, you can run the plugin from the command line or through the configuration file.
 
 ### Command line
 
@@ -29,7 +42,7 @@ Run Fluent Bit using a command similar to the following:
 fluent-bit -i netif -p interface=eth0 -o stdout
 ```
 
-Which returns something the following:
+Which returns output similar to the following:
 
 ```text
 ...
@@ -52,10 +65,12 @@ pipeline:
   inputs:
     - name: netif
       tag: netif
+      interface: eth0
       interval_sec: 1
       interval_nsec: 0
-      interface: eth0
-      
+      verbose: false
+      test_at_init: false
+
   outputs:
     - name: stdout
       match: '*'
@@ -68,9 +83,11 @@ pipeline:
 [INPUT]
   Name          netif
   Tag           netif
+  Interface     eth0
   Interval_Sec  1
   Interval_NSec 0
-  Interface     eth0
+  Verbose       false
+  Test_At_Init  false
 
 [OUTPUT]
   Name   stdout
@@ -80,6 +97,6 @@ pipeline:
 {% endtab %}
 {% endtabs %}
 
-Which calculates using the formula: `Total interval (sec) = Interval_Sec + (Interval_Nsec / 1000000000)`.
+Total interval (sec) = `interval_sec` + (`interval_nsec` / 1000000000)
 
-For example: `1.5s = 1s + 500000000ns`
+For example: `1.5s` = `1s` + `500000000ns`