out_loki: add label_map_path property (#895)

Signed-off-by: Takahiro Yamashita <[email protected]>

Signed-off-by: Takahiro Yamashita <[email protected]>

Author: nokute78

pipeline/outputs/loki.md

@@ -19,6 +19,7 @@ Be aware there is a separate Golang output plugin provided by [Grafana](https://
 | tenant\_id | Tenant ID used by default to push logs to Loki. If omitted or empty it assumes Loki is running in single-tenant mode and no X-Scope-OrgID header is sent. |  |
 | labels | Stream labels for API request. It can be multiple comma separated of strings specifying  `key=value` pairs. In addition to fixed parameters, it also allows to add custom record keys \(similar to `label_keys` property\). More details in the Labels section. | job=fluentbit |
 | label\_keys | Optional list of record keys that will be placed as stream labels. This configuration property is for records key only. More details in the Labels section. |  |
+| label\_map\_path | Specify the label map file path. The file defines how to extract labels from each record. More details in the Labels section. | |
 | remove\_keys | Optional list of keys to remove. | |
 | drop\_single\_key | If set to true and after extracting labels only a single key remains, the log line sent to Loki will be the value of that key in line\_format. | off |
 | line\_format | Format to use when flattening the record to a log line. Valid values are `json` or `key_value`. If set to `json`,  the log line sent to Loki will be the Fluent Bit record dumped as JSON. If set to `key_value`, the log line will be each item in the record concatenated together \(separated by a single space\) in the format. | json |
@@ -95,6 +96,48 @@ The following configuration examples generate the same Stream Labels:
     label_keys $sub['stream']
 ```
 
+the above configuration accomplish the same than this one:
+
+```text
+[OUTPUT]
+    name   loki
+    match  *
+    labels job=fluentbit, $sub['stream']
+```
+
+both will generate the following Streams label:
+
+```text
+job="fluentbit", stream="stdout"
+```
+
+### Using the `label_map_path` property
+
+The configuration property `label_map_path` is to read a JSON file that defines how to extract labels from each record.
+
+The file should contain a JSON object. Each keys define how to get label value from a nested record. Each values are used as label names.
+
+The following configuration examples generate the same Stream Labels:
+
+map.json:
+```json
+{
+    "sub": {
+           "stream": "stream"
+    }
+}
+```
+
+The following configuration examples generate the same Stream Labels:
+
+```text
+[OUTPUT]
+    name   loki
+    match  *
+    label_map_path /path/to/map.json
+```
+
+
 the above configuration accomplish the same than this one:
 
 ```text