Sandbox: Add Metrics rpc for controller

As a follow up change to adding a SandboxMetrics rpc to the core
sandbox service, the controller needed a corresponding rpc for CRI
and others to eventually implement.

This leaves the CRI (non-shim mode) controller unimplemented just to
have a change with the API addition to start.

Signed-off-by: Danny Canter <danny@dcantah.dev>

pkg/cri/sbserver/podsandbox/sandbox_stats.go

@@ -0,0 +1,29 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package podsandbox
+
+import (
+	"context"
+
+	"github.com/containerd/containerd/api/types"
+	"github.com/containerd/containerd/errdefs"
+)
+
+// TODO(dcantah): Implement metrics to be used for SandboxStats rpc.
+func (c *Controller) Metrics(ctx context.Context, sandboxID string) (*types.Metric, error) {
+	return nil, errdefs.ErrNotImplemented
+}