add example for http api prompt variables

Author: jsimpher

content/en/llm_observability/instrumentation/api.md

@@ -178,6 +178,26 @@ If the request is successful, the API responds with a 202 network code and an em
 | context_variable_keys | [string] | Variable keys that contain ground-truth or context content. Used for hallucination detection. |
 | tags | Dict[key (string), string] | Tags to attach to the prompt run. |
 
+**Note**: LLM Observability registers new versions of templates when the template or chat_template value is updated. If the input is expected to change between invocations, extract those dynamic parts into a variable.
+
+{{< code-block lang="json" >}}
+{
+  "id": "translation-prompt",
+  "chat_template": [
+    {
+      "role": "system",
+      "content": "You are a translation service. You translate to {{language}}."
+    }, {
+      "role": "user",
+      "content": "{{user_input}}"
+    }
+  ],
+  "variables": {
+    "language": "french",
+    "user_input": "<USER_INPUT_TEXT>"
+  }
+}
+{{< /code-block >}}
 
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