[dagster-airlift] Customizing dag proxying operator reference

docs/content/integrations/airlift/reference.mdx

@@ -123,3 +123,55 @@ defs = Definitions.merge(
     ),
 )
 ```
+
+### Customizing DAG proxying operator
+
+Similar to how we can customize the operator we construct on a per-dag basis, we can customize the operator we construct on a per-dag basis. We can use the `build_from_dag_fn` argument of `proxying_to_dagster` to provide a custom operator in place of the default.
+
+For example, let's take a look at the following custom operator which expects an API key to be provided as a variable:
+
+```python file=../../experimental/dagster-airlift/examples/tutorial-example/tutorial_example/snippets/custom_operator_examples/custom_dag_level_proxy.py
+from pathlib import Path
+
+import requests
+from airflow import DAG
+from airflow.utils.context import Context
+from dagster_airlift.in_airflow import BaseProxyDAGToDagsterOperator, proxying_to_dagster
+from dagster_airlift.in_airflow.proxied_state import load_proxied_state_from_yaml
+
+
+class CustomProxyToDagsterOperator(BaseProxyDAGToDagsterOperator):
+    def get_dagster_session(self, context: Context) -> requests.Session:
+        if "var" not in context:
+            raise ValueError("No variables found in context")
+        api_key = context["var"]["value"].get("my_api_key")
+        session = requests.Session()
+        session.headers.update({"Authorization": f"Bearer {api_key}"})
+        return session
+
+    def get_dagster_url(self, context: Context) -> str:
+        return "https://dagster.example.com/"
+
+    # This method controls how the operator is built from the dag.
+    @classmethod
+    def build_from_dag(cls, dag: DAG):
+        return CustomProxyToDagsterOperator(dag=dag, task_id="OVERRIDDEN")
+
+
+dag = DAG(
+    dag_id="custom_dag_level_proxy_example",
+)
+
+# At the end of your dag file
+proxying_to_dagster(
+    global_vars=globals(),
+    proxied_state=load_proxied_state_from_yaml(Path(__file__).parent / "proxied_state"),
+    build_from_dag_fn=CustomProxyToDagsterOperator.build_from_dag,
+)
+```
+
+`BaseProxyDAGToDagsterOperator` has three abstract methods which must be implemented:
+
+- `get_dagster_session`, which controls the creation of a valid session to access the Dagster graphql API.
+- `get_dagster_url`, which retrieves the domain at which the dagster webserver lives.
+- `build_from_dag`, which controls how the proxying task is constructed from the provided DAG.