treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/metropolis/node/kubernetes/metricsproxy/metricsproxy.go b/metropolis/node/kubernetes/metricsproxy/metricsproxy.go
index afa142d..1b98fa8 100644
--- a/metropolis/node/kubernetes/metricsproxy/metricsproxy.go
+++ b/metropolis/node/kubernetes/metricsproxy/metricsproxy.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package metricsproxy implements an authenticating proxy in front of the K8s
 // controller-manager and scheduler providing unauthenticated access to the
 // metrics via local ports