There are a couple of ways.
0 - Budgets programmatic notifications
I think flow from link below illustrates what you need.
https://cloud.google.com/billing/docs/how-to/budgets-programmatic-notifications
From budget alert > to pub/sub > to cloud function which you can program to do what you want including pinging slack/discord etc would have to use their APIs in cloud function. Example flow below from documentation:

1. Extracting your billing data to bigquery
https://cloud.google.com/billing/docs/how-to/export-data-bigquery
From there you could monitor billing by project and product, however I am not sure how instant it is, but you can imagine from there scheduling a BQ job or airflow to to check for statuses and ping pub/sub or cloud function to shut off what ever needs to be shut off or send messages like above
2. Set up budget alerts to yourself / other --- manual
https://cloud.google.com/billing/docs/how-to/budgets
You can set up alerts on when you are coming close to or exceeding your specified budget.
- In google cloud console go to billing
- Select your billing account (if multiple)
- Then go to Budgets & alerts.
I think you can publish these alerts to pub sub and go from there with whatever you need to shut down or migrate (like mentioned in 0.)
3. Budgets API
https://cloud.google.com/billing/docs/how-to/budget-api
There are some other cool things you can assemble from here for your solution.