pkg/prometheus: fix govet errors (#3916)

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

.golangci.yml

@@ -4,4 +4,5 @@ run:
 linters:
   disable-all: true
   enable:
-    - unused
+  - govet
+  - unused

pkg/prometheus/promcfg_test.go

@@ -1001,8 +1001,8 @@ func TestK8SSDConfigGeneration(t *testing.T) {
 			},
 			map[string]assets.BasicAuthCredentials{
 				"apiserver": {
-					"foo",
-					"bar",
+					Username: "foo",
+					Password: "bar",
 				},
 			},
 			`kubernetes_sd_configs: