code: func (ic *InstanceCount) Update(c context.Context, prefix string) error {
  	// Prefixes are globally unique, so we can use them as IDs.
  	ic.ID = prefix
  	ic.Computed = clock.Now(c).UTC()
  	ic.Prefix = prefix
  	if err := datastore.Put(c, ic); err != nil {
  		return errors.Annotate(err, "failed to store count").Err()
  	}
  	return nil
  }
docs: // Update updates metrics for all known counts of VMs for the given prefix.