Unit tests for Metrics Server k8sd feature (#691)

Metrics Server feature lacks unit test this PR implements tests for Metrics Server functionality.
KU-1515

src/k8s/pkg/k8sd/features/metrics-server/metrics_server_test.go

@@ -2,6 +2,7 @@ package metrics_server_test
 
 import (
 	"context"
+	"errors"
 	"testing"
 
 	"github.com/canonical/k8s/pkg/client/helm"
@@ -14,44 +15,73 @@ import (
 )
 
 func TestApplyMetricsServer(t *testing.T) {
+
+	helmErr := errors.New("failed to apply")
 	for _, tc := range []struct {
 		name        string
 		config      types.MetricsServer
 		expectState helm.State
+		helmError   error
 	}{
 		{
-			name: "Enable",
+			name: "EnableWithoutHelmError",
+			config: types.MetricsServer{
+				Enabled: utils.Pointer(true),
+			},
+			expectState: helm.StatePresent,
+			helmError:   nil,
+		},
+		{
+			name: "DisableWithoutHelmError",
+			config: types.MetricsServer{
+				Enabled: utils.Pointer(false),
+			},
+			expectState: helm.StateDeleted,
+			helmError:   nil,
+		},
+		{
+			name: "EnableWithHelmError",
 			config: types.MetricsServer{
 				Enabled: utils.Pointer(true),
 			},
 			expectState: helm.StatePresent,
+			helmError:   helmErr,
 		},
 		{
-			name: "Disable",
+			name: "DisableWithHelmError",
 			config: types.MetricsServer{
 				Enabled: utils.Pointer(false),
 			},
 			expectState: helm.StateDeleted,
+			helmError:   helmErr,
 		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			g := NewWithT(t)
-			h := &helmmock.Mock{}
+			h := &helmmock.Mock{
+				ApplyErr: tc.helmError,
+			}
 			s := &snapmock.Snap{
 				Mock: snapmock.Mock{
 					HelmClient: h,
 				},
 			}
 
 			status, err := metrics_server.ApplyMetricsServer(context.Background(), s, tc.config, nil)
-			g.Expect(err).ToNot(HaveOccurred())
+			if tc.helmError == nil {
+				g.Expect(err).ToNot(HaveOccurred())
+			} else {
+				g.Expect(err).To(HaveOccurred())
+			}
 
 			g.Expect(h.ApplyCalledWith).To(ConsistOf(SatisfyAll(
 				HaveField("Chart.Name", Equal("metrics-server")),
 				HaveField("Chart.Namespace", Equal("kube-system")),
 				HaveField("State", Equal(tc.expectState)),
 			)))
-			if tc.config.GetEnabled() {
+			if errors.Is(tc.helmError, helmErr) {
+				g.Expect(status.Message).To(ContainSubstring(helmErr.Error()))
+			} else if tc.config.GetEnabled() {
 				g.Expect(status.Message).To(Equal("enabled"))
 			} else {
 				g.Expect(status.Message).To(Equal("disabled"))