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/cloud/bmaas/bmdb/metrics/metrics.go b/cloud/bmaas/bmdb/metrics/metrics.go
index 509827b..242ac2d 100644
--- a/cloud/bmaas/bmdb/metrics/metrics.go
+++ b/cloud/bmaas/bmdb/metrics/metrics.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package metrics implements a Prometheus metrics submission interface for BMDB
 // client components. A Metrics object can be attached to a BMDB object, which
 // will make all BMDB sessions/transactions/work statistics be submitted to that