adding a complete copy paste example for user in add_entries

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

@@ -222,3 +222,146 @@ The processed event will contain the following data:
 ```json
 {"message": ["hello", "world"]}
 ```
+
+## Complete example
+
+The following pipeline is used to:
+
+1. Add `app_id` using a format string ${app}-${env}.
+2. Add `message_len` with `length(/message)`.
+3. Add metadata key `msg_len_meta` with `length(/message)`.
+4. If `/metric/name` and `/metric/value` exist, create a new field named after `/metric/name` and set its value to `/metric/value`.
+5. If `/level == "error"`, add `severity: "high"`.
+6. Add `"ingested"` to `tags`.
+7. Set `env_normalized: "prod"` and overwrite it if it already exists.
+
+```yaml
+example-pipeline:
+  source:
+    http:
+      port: 2021
+      path: /events
+      ssl: false
+
+  processor:
+    - add_entries:
+        entries:
+          - key: "app_id"
+            format: "${app}-${env}"
+
+          - key: "message_len"
+            value_expression: "length(/message)"
+
+          - metadata_key: "msg_len_meta"
+            value_expression: "length(/message)"
+
+          # dynamic key from the event, only when both metric fields exist
+          - key: "${/metric/name}"
+            value_expression: "/metric/value"
+            add_when: "/metric/name != null and /metric/value != null"
+
+          # set severity ONLY on error level
+          - key: "severity"
+            value: "high"
+            add_when: '/level == "error"'
+
+          # append behavior: if tags already exists, it becomes/extends an array
+          - key: "tags"
+            value: "ingested"
+            append_if_key_exists: true
+
+          # overwrite behavior
+          - key: "env_normalized"
+            value: "prod"
+            overwrite_if_key_exists: true
+
+  sink:
+    - opensearch:
+        hosts: ["https://opensearch:9200"]
+        insecure: true
+        username: admin
+        password: "admin_pass"
+        index_type: custom
+        index: "example-%{yyyy.MM.dd}"
+```
+{% include copy.html %}
+
+You can test this pipeline by executing the following command:
+
+```bash
+curl -sS -X POST "http://localhost:2021/events" \
+  -H "Content-Type: application/json" \
+  -d '[
+        {"app":"shop","env":"dev","message":"hello","level":"info","metric":{"name":"cpu","value":42}},
+        {"app":"shop","env":"prod","message":"boom","level":"error"},
+        {"app":"api","env":"stage","message":"hi","level":"warn","metric":{"name":"mem","value":2048},"tags":"pretag"}
+      ]'
+```
+{% include copy.html %}
+
+Once these documents are processed, the index `example-2025.10.10` will contain the following documents:
+
+```json
+"hits": [
+  {
+    "_index": "example-2025.10.10",
+    "_id": "BnvWzpkBTMZ443JmHuHI",
+    "_score": 1,
+    "_source": {
+      "app": "shop",
+      "env": "dev",
+      "message": "hello",
+      "level": "info",
+      "metric": {
+        "name": "cpu",
+        "value": 42
+      },
+      "app_id": "shop-dev",
+      "message_len": 5,
+      "cpu": 42,
+      "tags": "ingested",
+      "env_normalized": "prod"
+    }
+  },
+  {
+    "_index": "example-2025.10.10",
+    "_id": "B3vWzpkBTMZ443JmHuHI",
+    "_score": 1,
+    "_source": {
+      "app": "shop",
+      "env": "prod",
+      "message": "boom",
+      "level": "error",
+      "app_id": "shop-prod",
+      "message_len": 4,
+      "severity": "high",
+      "tags": "ingested",
+      "env_normalized": "prod"
+    }
+  },
+  {
+    "_index": "example-2025.10.10",
+    "_id": "CHvWzpkBTMZ443JmHuHI",
+    "_score": 1,
+    "_source": {
+      "app": "api",
+      "env": "stage",
+      "message": "hi",
+      "level": "warn",
+      "metric": {
+        "name": "mem",
+        "value": 2048
+      },
+      "tags": [
+        "pretag",
+        "ingested"
+      ],
+      "app_id": "api-stage",
+      "message_len": 2,
+      "mem": 2048,
+      "env_normalized": "prod"
+    }
+  }
+]
+
+```