osbase/supervisor: implement Metrics API

This is a base building block for exporting per-DN/runnable status from
the supervisor into an external system. A sample implementation is
provided which can be used in simple debug facilities to inspect the
current supervision tree.

A follow-up change will use the same API to implement Prometheus
metrics.

Change-Id: I0d586b03a397a3ccf8dac2d8043b9dd2f319be4e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3290
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/osbase/supervisor/BUILD.bazel b/osbase/supervisor/BUILD.bazel
index 252b3b7..1997f45 100644
--- a/osbase/supervisor/BUILD.bazel
+++ b/osbase/supervisor/BUILD.bazel
@@ -4,6 +4,7 @@
     name = "supervisor",
     srcs = [
         "supervisor.go",
+        "supervisor_metrics.go",
         "supervisor_node.go",
         "supervisor_processor.go",
         "supervisor_support.go",