Add warning for plugin configs with unknown fields

Signed-off-by: Derek McGowan <derek@mcg.dev>

services/server/config/config_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/stretchr/testify/assert"
 
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/log/logtest"
 )
 
 func TestMergeConfigs(t *testing.T) {
@@ -191,6 +192,7 @@ imports = ["data1.toml", "data2.toml"]
 }
 
 func TestDecodePlugin(t *testing.T) {
+	ctx := logtest.WithT(context.Background(), t)
 	data := `
 version = 2
 [plugins."io.containerd.runtime.v1.linux"]
@@ -208,7 +210,7 @@ version = 2
 	assert.NoError(t, err)
 
 	pluginConfig := map[string]interface{}{}
-	_, err = out.Decode(&plugin.Registration{Type: "io.containerd.runtime.v1", ID: "linux", Config: &pluginConfig})
+	_, err = out.Decode(ctx, &plugin.Registration{Type: "io.containerd.runtime.v1", ID: "linux", Config: &pluginConfig})
 	assert.NoError(t, err)
 	assert.Equal(t, true, pluginConfig["shim_debug"])
 }