osbase/supervisor: use MustRegister for metrics registration

Change-Id: I4321c626f210bea025ab27bfecf783425f1482b5
Reviewed-on: https://review.monogon.dev/c/monogon/+/3703
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/osbase/supervisor/BUILD.bazel b/osbase/supervisor/BUILD.bazel
index 9dfda5a..2fc6634 100644
--- a/osbase/supervisor/BUILD.bazel
+++ b/osbase/supervisor/BUILD.bazel
@@ -19,6 +19,7 @@
         "//osbase/logtree",
         "@com_github_cenkalti_backoff_v4//:backoff",
         "@com_github_prometheus_client_golang//prometheus",
+        "@com_github_prometheus_client_golang//prometheus/promauto",
         "@org_golang_google_grpc//:grpc",
     ],
 )