plugin/metrics: fix failed reload (#2816)

Fix metrics endpoint on a failed reload, follows the same lines as the
previous PRs, see for e.g. 076b8d4f. Test with a Corefile with 2 server
blocks and metrics enabled and then introducing a syntax error:

~~~
[ERROR] Restart failed: Corefile:5 - Error during parsing: Unknown directive 'jfkdjk'
[ERROR] SIGUSR1: starting with listener file descriptors: Corefile:5 - Error during parsing: Unknown directive 'jfkdjk'
~~~

And then curl-ing the metrics endpoint.

See #2659 and as this is the last one.

Fixes: #2659

Getting this all right turns out to be tricky, also it's not easy
testable which is something I should fix.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/metrics/metrics.go

@@ -137,9 +137,7 @@ func (m *Metrics) stopServer() error {
 }
 
 // OnFinalShutdown tears down the metrics listener on shutdown and restart.
-func (m *Metrics) OnFinalShutdown() error {
-	return m.stopServer()
-}
+func (m *Metrics) OnFinalShutdown() error { return m.stopServer() }
 
 func keys(m map[string]struct{}) []string {
 	sx := []string{}