As a workaround you can use another channel.
For example you can configure the alerting policy via email, and send the notification to the Email address of your Microsoft Teams Channel.
In order to forward / send an Email to an MS Teams Channel, you need to know what is the Email address of your Team Channel, and is pretty simple, just find the Channel that you’d like to Email, find the ellipsis and then select “Get Email address”,check this image for reference.
Once you’ve clicked “Get Email address”, just make a copy of it and you are ready to sent your Stack Driver alerts directly to your MS Teams Chanel using Email alerting configuration.
You can read this "How to send an email to a channel in Teams" guide on Microsoft Support site.
Other Workaround are you can use a service in the middle that catch the Stackdriver message (JSON) and converts it to MS Teams format and then send it again to the desired destination, but it maybe involves some development work to your team.
Also i can see Google have an active Feature request about your question.
You can track the evolution here and I would recommend you to ‘star it’ in order to receive updates on its progress.
I hope the information I have provided will be helpful to you. =)