[DOC] Add drop processor (opensearch-project#5767)

* Add drop processor doc to address content gap

Signed-off-by: Melissa Vagi <[email protected]>

* Address tech review feedback

Signed-off-by: Melissa Vagi <[email protected]>

* Address tech review changes

Signed-off-by: Melissa Vagi <[email protected]>

* Delete _ingest-pipelines/processors/date.md

Signed-off-by: Melissa Vagi <[email protected]>

Signed-off-by: Melissa Vagi <[email protected]>

* Revert "Delete _ingest-pipelines/processors/date.md"

This reverts commit 73296f5.

* Update _ingest-pipelines/processors/drop.md

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Melissa Vagi <[email protected]>

* Update _ingest-pipelines/processors/drop.md

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Melissa Vagi <[email protected]>

* Update _ingest-pipelines/processors/drop.md

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Melissa Vagi <[email protected]>

* Update _ingest-pipelines/processors/drop.md

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Melissa Vagi <[email protected]>

* Update _ingest-pipelines/processors/drop.md

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Melissa Vagi <[email protected]>

* Update drop.md

Signed-off-by: Melissa Vagi <[email protected]>

Signed-off-by: Melissa Vagi <[email protected]>

---------

Signed-off-by: Melissa Vagi <[email protected]>
Co-authored-by: Nathan Bower <[email protected]>

_ingest-pipelines/processors/date.md

@@ -11,7 +11,7 @@ redirect_from:
 
 The `date` processor is used to parse dates from document fields and to add the parsed data to a new field. By default, the parsed data is stored in the `@timestamp` field. 
 
-## Syntax
+## Syntax example
 
 The following is the syntax for the `date` processor:
 

_ingest-pipelines/processors/drop.md

@@ -0,0 +1,123 @@
+---
+layout: default
+title: Drop
+parent: Ingest processors
+nav_order: 70
+---
+
+# Drop processor
+
+The `drop` processor is used to discard documents without indexing them. This can be useful for preventing documents from being indexed based on certain conditions. For example, you might use a `drop` processor to prevent documents that are missing important fields or contain sensitive information from being indexed. 
+
+The `drop` processor does not raise any errors when it discards documents, making it useful for preventing indexing problems without cluttering your OpenSearch logs with error messages.
+
+## Syntax example
+
+The following is the syntax for the `drop` processor:
+
+```json
+{
+  "drop": {
+    "if": "ctx.foo == 'bar'"
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Configuration parameters
+
+The following table lists the required and optional parameters for the `drop` processor.
+
+Parameter | Required | Description |
+|-----------|-----------|-----------|
+`description`  | Optional  | A brief description of the processor.  |
+`if` | Optional | A condition for running the processor. |
+`ignore_failure` | Optional | If set to `true`, failures are ignored. Default is `false`. See [Handling pipeline failures]({{site.url}}{{site.baseurl}}/ingest-pipelines/pipeline-failures/) for more information. |
+`on_failure` | Optional | A list of processors to run if the processor fails. See [Handling pipeline failures]({{site.url}}{{site.baseurl}}/ingest-pipelines/pipeline-failures/) for more information. |
+`tag` | Optional | An identifier tag for the processor. Useful for distinguishing between processors of the same type when debugging. |
+
+## Using the processor
+
+Follow these steps to use the processor in a pipeline.
+
+**Step 1: Create a pipeline**
+
+The following query creates a pipeline, named `drop-pii`, that uses the `drop` processor to prevent a document containing personally identifiable information (PII) from being indexed:
+
+```json
+PUT /_ingest/pipeline/drop-pii
+{
+  "description": "Pipeline that prevents PII from being indexed",
+  "processors": [
+    {
+      "drop": {
+        "if" : "ctx.user_info.contains('password') || ctx.user_info.contains('credit card')"
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+**Step 2 (Optional): Test the pipeline**
+
+It is recommended that you test your pipeline before ingesting documents.
+{: .tip}
+
+To test the pipeline, run the following query:
+
+```json
+POST _ingest/pipeline/drop-pii/_simulate
+{
+  "docs": [
+    {
+      "_index": "testindex1",
+      "_id": "1",
+      "_source": {
+        "user_info": "Sensitive information including credit card"
+      }
+    }
+  ]
+}
+```
+{% include copy-curl.html %}
+
+#### Response
+
+The following example response confirms that the pipeline is working as expected (the document has been dropped):
+
+```json
+{
+  "docs": [
+    null
+  ]
+}
+```
+{% include copy-curl.html %}
+
+**Step 3: Ingest a document**
+
+The following query ingests a document into an index named `testindex1`:
+
+```json
+PUT testindex1/_doc/1?pipeline=drop-pii
+{
+  "user_info": "Sensitive information including credit card"
+}
+```
+{% include copy-curl.html %}
+
+The following response confirms that the document with the ID of `1` was not indexed: 
+
+{
+  "_index": "testindex1",
+  "_id": "1",
+  "_version": -3,
+  "result": "noop",
+  "_shards": {
+    "total": 0,
+    "successful": 0,
+    "failed": 0
+  }
+}
+{% include copy-curl.html %}