pipeline: inputs: mqtt: update mqtt input config with buffer_size defaults and payload_key examples (#2283)

* pipeline: inputs: mqtt: update mqtt input config with buffer_size defaults and payload_key examples. Fixes #2282.

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

* pipeline: inputs: mqtt: updated description of payload_key as per review comments.

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

---------

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

Author: eschabell

pipeline/inputs/mqtt.md

@@ -6,12 +6,22 @@ The _MQTT_ input plugin retrieves messages and data from MQTT control packets ov
 
 The plugin supports the following configuration parameters:
 
-| Key           | Description                                                                                             | Default   |
-|:--------------|:--------------------------------------------------------------------------------------------------------|:----------|
-| `Listen`      | Listener network interface.                                                                             | `0.0.0.0` |
-| `Port`        | TCP port where listening for connections.                                                               | `1883`    |
-| `Payload_Key` | Specify the key where the payload key/value will be preserved.                                          | _none_    |
-| `Threaded`    | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs). | `false`   |
+| Key           | Description                                                                                             | Default      |
+|:--------------|:--------------------------------------------------------------------------------------------------------|:-------------|
+| `buffer_size` | Maximum payload size (in bytes) for a single MQTT message.                                              | `2048`       |
+| `listen`      | Listener network interface.                                                                             | `0.0.0.0`    |
+| `payload_key` | Field name where the MQTT message payload will be stored in the output record.                          | _none_       |
+| `port`        | TCP port where listening for connections.                                                               | `1883`       |
+| `threaded`    | Indicates whether to run this input in its own [thread](../../administration/multithreading.md#inputs). | `false`      |
+
+Notes:
+- `buffer_size` defaults to `2048` bytes; messages larger than this limit are dropped.
+- Defaults for `listen` and `port` are `0.0.0.0` and `1883`, so you can omit them if you want the standard MQTT listener.
+- Payloads are expected to be JSON maps; non-JSON payloads will fail to parse.
+
+### TLS / SSL
+
+The MQTT input plugin supports TLS/SSL. For the available options and guidance, see [Transport Security](../../administration/transport-security.md).
 
 ## Get started
 
@@ -55,6 +65,7 @@ pipeline:
       tag: data
       listen: 0.0.0.0
       port: 1883
+      payload_key: payload
 
   outputs:
     - name: stdout
@@ -70,6 +81,7 @@ pipeline:
   Tag    data
   Listen 0.0.0.0
   Port   1883
+  Payload_Key payload
 
 [OUTPUT]
   Name   stdout