You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: upgrade.go
    path of file: ./repos/juju/caas/kubernetes/provider
    
    the code of the file until where you have to start completion: // Copyright 2017 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package provider

import (
	"context"
	"fmt"

	"github.com/juju/errors"
	"github.com/juju/names/v5"
	"github.com/juju/version/v2"
	apps "k8s.io/api/apps/v1"
	core "k8s.io/api/core/v1"
	k8s
    
func (k *kubernetesClient) Upgrade(agentTag string, vers version.Number) error {
	tag, err := names.ParseTag(agentTag)
	if err != nil {
		return errors.Annotate(err, "parsing agent tag to upgrade")
	}

	logger.Infof("handling upgrade request for tag %q to %s", tag, vers.String())

	switch tag.Kind() {
	case names.MachineTagKind:
	case names.ControllerAgentTagKind:
		return k.upgradeController(vers)
	case names.ApplicationTagKind:
		return k.upgradeOperator(tag, vers)
	case names.ModelTagKind:
		return k.upgradeModelOperator(tag, vers)
	case names.UnitTagKind:
		// Sidecar charms don't have an upgrade step.
		// See PR 14974
		return nil
	}
	return errors.NotImplementedf("k8s upgrade for agent tag %q", agentTag)
}