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/setup_test.go

@@ -22,7 +22,7 @@ func TestPrometheusParse(t *testing.T) {
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.input)
-		m, err := prometheusParse(c)
+		m, err := parse(c)
 		if test.shouldErr && err == nil {
 			t.Errorf("Test %v: Expected error but found nil", i)
 			continue