docs: describe event and context in Pub/Sub example (#5862)

* docs: describe event and context in Pub/Sub example

* mock context resource as well

functions/helloworld/main.py

@@ -70,16 +70,27 @@ def hello_pubsub(event, context):
     """Background Cloud Function to be triggered by Pub/Sub.
     Args:
          event (dict):  The dictionary with data specific to this type of
-         event. The `data` field contains the PubsubMessage message. The
-         `attributes` field will contain custom attributes if there are any.
-         context (google.cloud.functions.Context): The Cloud Functions event
-         metadata. The `event_id` field contains the Pub/Sub message ID. The
-         `timestamp` field contains the publish time.
+                        event. The `@type` field maps to
+                         `type.googleapis.com/google.pubsub.v1.PubsubMessage`.
+                        The `data` field maps to the PubsubMessage data
+                        in a base64-encoded string. The `attributes` field maps
+                        to the PubsubMessage attributes if any is present.
+         context (google.cloud.functions.Context): Metadata of triggering event
+                        including `event_id` which maps to the PubsubMessage
+                        messageId, `timestamp` which maps to the PubsubMessage
+                        publishTime, `event_type` which maps to
+                        `google.pubsub.topic.publish`, and `resource` which is
+                        a dictionary that describes the service API endpoint
+                        pubsub.googleapis.com, the triggering topic's name, and
+                        the triggering event type
+                        `type.googleapis.com/google.pubsub.v1.PubsubMessage`.
+    Returns:
+        None. The output is written to Cloud Logging.
     """
     import base64
 
-    print("""This Function was triggered by messageId {} published at {}
-    """.format(context.event_id, context.timestamp))
+    print("""This Function was triggered by messageId {} published at {} to {}
+    """.format(context.event_id, context.timestamp, context.resource["name"]))
 
     if 'data' in event:
         name = base64.b64decode(event['data']).decode('utf-8')

functions/helloworld/sample_pubsub_test.py

@@ -23,6 +23,11 @@
 mock_context = mock.Mock()
 mock_context.event_id = '617187464135194'
 mock_context.timestamp = '2019-07-15T22:09:03.761Z'
+mock_context.resource = {
+    'name': 'projects/my-project/topics/my-topic',
+    'service': 'pubsub.googleapis.com',
+    'type': 'type.googleapis.com/google.pubsub.v1.PubsubMessage',
+}
 
 
 def test_print_hello_world(capsys):