docs: add asyncpg sample usage

tests/system/test_asyncpg_connection.py

@@ -14,34 +14,86 @@
 
 import os
 
+# [START alloydb_sqlalchemy_connect_async_connector]
 import asyncpg
 import pytest
 import sqlalchemy
-from sqlalchemy.ext.asyncio import create_async_engine
 
 from google.cloud.alloydb.connector import AsyncConnector
 
 
-@pytest.mark.asyncio
-async def test_connection_with_asyncpg() -> None:
-    async with AsyncConnector() as connector:
-
-        async def getconn() -> asyncpg.Connection:
-            conn: asyncpg.Connection = await connector.connect(
-                os.environ["ALLOYDB_INSTANCE_URI"],
-                "asyncpg",
-                user=os.environ["ALLOYDB_USER"],
-                password=os.environ["ALLOYDB_PASS"],
-                db=os.environ["ALLOYDB_DB"],
-            )
-            return conn
+async def create_sqlalchemy_engine(
+    inst_uri: str,
+    user: str,
+    password: str,
+    db: str,
+) -> (sqlalchemy.engine.AsyncEngine, AsyncConnector):
+    """Creates a connection pool for an AlloyDB instance and returns the pool
+    and the connector. Callers are responsible for closing the pool and the
+    connector.
+
+    A sample invocation looks like:
+
+        engine, connector = create_sqlalchemy_engine(
+                inst_uri,
+                user,
+                password,
+                db,
+        )
+        with engine.connect() as conn:
+            time = conn.execute(sqlalchemy.text("SELECT NOW()")).fetchone()
+            conn.commit()
+            curr_time = time[0]
+            # do something with query result
+            connector.close()
+
+    Args:
+        instance_uri (str):
+            The instance URI specifies the instance relative to the project,
+            region, and cluster. For example:
+            "projects/my-project/locations/us-central1/clusters/my-cluster/instances/my-instance"
+        user (str):
+            The database user name, e.g., postgres
+        password (str):
+            The database user's password, e.g., secret-password
+        db_name (str):
+            The name of the database, e.g., mydb
+    """
+    connector = AsyncConnector()
+
+    async def getconn() -> asyncpg.Connection:
+        conn: asyncpg.Connection = await connector.connect(
+            inst_uri,
+            "asyncpg",
+            user=user,
+            password=password,
+            db=db,
+        )
+        return conn
 
     # create SQLAlchemy connection pool
-    pool = create_async_engine(
+    engine = await sqlalchemy.create_async_engine(
         "postgresql+asyncpg://",
         async_creator=getconn,
         execution_options={"isolation_level": "AUTOCOMMIT"},
     )
+    engine.dialect.description_encoding = None
+    return engine, connector
+
+
+# [END alloydb_sqlalchemy_connect_async_connector]
+
+
+@pytest.mark.asyncio
+async def test_connection_with_asyncpg() -> None:
+    """Basic test to get time from database."""
+    inst_uri = os.environ["ALLOYDB_INSTANCE_URI"]
+    user = os.environ["ALLOYDB_USER"]
+    password = os.environ["ALLOYDB_PASS"]
+    db = os.environ["ALLOYDB_DB"]
+
+    pool = await create_sqlalchemy_engine(inst_uri, user, password, db)
+
     async with pool.connect() as conn:
         res = (await conn.execute(sqlalchemy.text("SELECT 1"))).fetchone()
         assert res[0] == 1