Add-Entries Processor (#6385)

* Add-Entries Processor

Signed-off-by: shaavanga <prathyuvanga@gmail.com>

* Add-Entries Processor

Signed-off-by: shaavanga <prathyuvanga@gmail.com>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

* Update _data-prepper/pipelines/configuration/processors/add-entries.md

Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Signed-off-by: Prathyusha Vangala <157630736+shaavanga@users.noreply.github.com>

* removing backticks

Signed-off-by: shaavanga <prathyuvanga@gmail.com>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

* Update _data-prepper/pipelines/configuration/processors/add-entries.md

Co-authored-by: Nathan Bower <nbower@amazon.com>
Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Nathan Bower <nbower@amazon.com>
Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

---------

Signed-off-by: shaavanga <prathyuvanga@gmail.com>
Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Signed-off-by: Prathyusha Vangala <157630736+shaavanga@users.noreply.github.com>
Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Co-authored-by: Nathan Bower <nbower@amazon.com>

_data-prepper/pipelines/configuration/processors/add-entries.md

@@ -18,6 +18,10 @@ You can configure the `add_entries` processor with the following options.
 | :--- | :--- | :--- |
 | `entries` | Yes | A list of entries to add to an event. |
 | `key` | Yes | The key of the new entry to be added. Some examples of keys include `my_key`, `myKey`, and `object/sub_Key`. |
+| `metadata_key` | Yes | The key for the new metadata attribute. The argument must be a literal string key and not a JSON Pointer. Either one string key or `metadata_key` is required. |
+| `format` | No | A format string to use as the value of the new entry, for example, `${key1}-${key2}`, where `key1` and `key2` are existing keys in the event. Required if neither `value` nor `value_expression` is specified. |
+| `value_expression` | No | An expression string to use as the value of the new entry. For example, `/key` is an existing key in the event with a type of either a number, a string, or a Boolean. Expressions can also contain functions returning number/string/integer. For example, `length(/key)` will return the length of the key in the event when the key is a string. For more information about keys, see [Expression syntax](https://opensearch.org/docs/latest/data-prepper/pipelines/expression-syntax/).  |
+| `add_when` | No | A [conditional expression](https://opensearch.org/docs/latest/data-prepper/pipelines/expression-syntax/), such as `/some-key == "test"'`, that will be evaluated to determine whether the processor will be run on the event. |
 | `value` | Yes | The value of the new entry to be added. You can use the following data types: strings, Booleans, numbers, null, nested objects, and arrays. |
 | `overwrite_if_key_exists` | No | When set to `true`, the existing value is overwritten if `key` already exists in the event. The default value is `false`. |
 
@@ -36,6 +40,9 @@ pipeline:
         - key: "newMessage"
           value: 3
           overwrite_if_key_exists: true
+        - metadata_key: myMetadataKey
+          value_expression: 'length("newMessage")'
+          add_when: '/some_key == "test"'
   sink:
 ```
 {% include copy.html %}