Dyno Load errors and threshold alerts

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku dynos

### Tell us about what you're trying to solve. What challenges are you facing?

My current application is CPU constrained as opposed to memory constrained, and I noticed the Metrics tooling around "dyno load" is not as advanced as the memory stuff, presumably because being memory constrained is more common.

I wonder if it's worth considering adding a new error code as we have for memory errors: https://devcenter.heroku.com/articles/error-codes#r14-memory-quota-exceeded

I'd also love to see threshold alerts based on memory and dyno load as well. (For example, if my app's max dyno load is over 1.0 for 5 minutes, alert me.)

Thanks for your consideration!