plugin/metrics: Switch to using promhttp instead of deprecated Handler (#1312)

prometheus.Handler is deprecated according to the godoc for the package so
instead we're using promhttp.

Additionally, we are exposing the Registry that metrics is using so other
plugins that are not inside of coredns can read the registry. Otherwise, if
we kept using the Default one, there's no way to access that from outside
of the coredns repo since it is vendored.

vendor/github.com/imdario/mergo/testdata/license.yml

@@ -0,0 +1,4 @@
+import: ../../../../fossene/db/schema/thing.yml
+fields:
+    site: string
+    author: root

vendor/github.com/imdario/mergo/testdata/thing.yml

@@ -0,0 +1,6 @@
+fields:
+    id: int
+    name: string
+    parent: ref "datu:thing"
+    status: enum(draft, public, private)
+    author: updater