test(spanner): await database calls in async snippets (#17627)

Awaited `instance.database()` calls in async snippets to avoid
AttributeError.

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Fixes #<issue_number_goes_here> 🦕

Author: sakthivelmanii

packages/google-cloud-spanner/samples/samples/async_snippets.py

@@ -26,7 +26,7 @@ async def async_create_client(instance_id, database_id):
     """Instantiates an asynchronous Spanner client."""
     spanner_client = AsyncClient()
     instance = spanner_client.instance(instance_id)
-    database = instance.database(database_id)
+    database = await instance.database(database_id)
 
     print("Async Spanner client instantiated successfully.")
     return database
@@ -40,7 +40,7 @@ async def async_query_data(instance_id, database_id):
     """Queries sample data from the database using asynchronous SQL."""
     spanner_client = AsyncClient()
     instance = spanner_client.instance(instance_id)
-    database = instance.database(database_id)
+    database = await instance.database(database_id)
 
     async with database.snapshot() as snapshot:
         results = await snapshot.execute_sql(
@@ -59,7 +59,7 @@ async def async_insert_data(instance_id, database_id):
     """Inserts sample data into the database using DML asynchronously."""
     spanner_client = AsyncClient()
     instance = spanner_client.instance(instance_id)
-    database = instance.database(database_id)
+    database = await instance.database(database_id)
 
     async def insert_singers(transaction):
         dml = (
@@ -81,7 +81,7 @@ async def async_read_write_transaction(instance_id, database_id):
     """Performs an asynchronous read-write transaction."""
     spanner_client = AsyncClient()
     instance = spanner_client.instance(instance_id)
-    database = instance.database(database_id)
+    database = await instance.database(database_id)
 
     async def update_singer_lastname(transaction):
         # Retrieve current name
@@ -110,7 +110,7 @@ async def async_read_only_transaction(instance_id, database_id):
     """Performs an asynchronous read-only transaction."""
     spanner_client = AsyncClient()
     instance = spanner_client.instance(instance_id)
-    database = instance.database(database_id)
+    database = await instance.database(database_id)
 
     async with database.snapshot() as snapshot:
         # Execute a read using standard KeySet