Billing for server-side batch changes on managed instances

Created by: malomarrec

Also see RFC 563 PRIVATE WIP: Executors pricing and billing for managed instances and cloud

This is still work in progress, but here's a quick plan

• validate pricing model with customers (pay as you go or billed in arrears) (notes in RFC 563)
• validate usage pattern with a free tier Beta
• add compute usage tracking
• and a reporting dashboard available to the customer
• create a way for a CE to managed the consumption available to a customer
• create a way for a customer to set their max number of executors and per-user limits