[EventGrid] async EventGridPublisherClient does not close session

[swathipil]

• Package Name: azure-eventgrid
• Package Version: 2.0.0b5
• Operating System: Windows 10
• Python Version: 3.7

Describe the bug
Async EventGridPublisherClient does not close the client using aexit. Need to fix async samples to remove warning.

To Reproduce
Steps to reproduce the behavior:

1. Run the following sample, adding the right credentials to environment variables as needed:

# --------------------------------------------------------------------------
# Copyright (c) Microsoft Corporation. All rights reserved.
# Licensed under the MIT License. See License.txt in the project root for
# license information.
# --------------------------------------------------------------------------
"""
FILE: cs1_publish_custom_events_to_a_topic_async.py
DESCRIPTION:
    These samples demonstrate sending an EventGrid Event asynchronously.
USAGE:
    python cs1_publish_custom_events_to_a_topic_async.py
    Set the environment variables with your own values before running the sample:
    1) EG_ACCESS_KEY - The access key of your eventgrid account.
    2) EG_TOPIC_HOSTNAME - The topic hostname. Typically it exists in the format
    "<YOUR-TOPIC-NAME>.<REGION-NAME>.eventgrid.azure.net".
"""
import os
import asyncio

from azure.eventgrid.aio import EventGridPublisherClient
from azure.eventgrid import EventGridEvent, CloudEvent
from azure.core.credentials import AzureKeyCredential

async def publish():
	topic_key = os.environ["EG_ACCESS_KEY"]
	topic_hostname = os.environ["EG_TOPIC_HOSTNAME"]

	credential = AzureKeyCredential(topic_key)
	client = EventGridPublisherClient(topic_hostname, credential)

	await client.send([
		EventGridEvent(
			event_type="Contoso.Items.ItemReceived",
			data={
				"itemSku": "Contoso Item SKU #1"
			},
			subject="Door1",
			data_version="2.0"
		)
	])

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(publish())

Expected behavior
Samples should run without warning.

Warning

Unclosed client session
client_session: <aiohttp.client.ClientSession object at 0x000002414047DEF0>
Unclosed connector
connections: ['[(<aiohttp.client_proto.ResponseHandler object at 0x00000241404BD388>, 119561.203)]']connector: <aiohttp.connector.TCPConnector object at 0x000002414047DEB8>