v0.21.0

Relatively small release, with significant changes to per-VM metrics,
and some other minor improvements elsewhere.

Features:

- agent: Include autoscaling bounds annotation in per-VM metrics (#695)

Breaking changes:

- agent: Separate per-VM memory and cpu metrics (#684)
  - The `autoscaling_vm_resources` metric is now split into
    `autoscaling_vm_cpu_cores` and `autoscaling_vm_memory_bytes`, with
    the "resource" label removed.

No protocol changes.

Fixes:

- neonvm/whereabouts: Require amd64 to fix issues with ARM nodes (#691)
  - Previously released as v0.20.0-patch1
- plugin: Fix nil deref on failed ExtractVmInfo on VM update (#690)
  - Previously released as v0.20.0-patch1
- agent: Clarify some logs about monitor requests (#687)
- Revert neonvm/runner: Treat guest-side disk flush requests as no-ops (#628)
  - This was previously released as v0.20.0-patch2.

Other changes:

- vm-builder: Set vector scrape interval from 15s to 1s (#650)

Upgrade path from v0.20.0:

- Components can be deployed in any order.
- The per-VM metrics exposed by the autoscaler-agent have changed in a
  backwards-incompatible way. Any usage must be updated.