core: hide registerHandler (#964)

* core: hide registerHandler

Remove RegisterHandler and just make it implicit when we look at the
handler compilation step.

* Rename GetHandler to just Handler

Update callers and make auto check Hander in OnStartup.

* Up test coverage in erratic

* up test coverage

middleware/metrics/setup_test.go

@@ -18,6 +18,7 @@ func TestPrometheusParse(t *testing.T) {
 		// fails
 		{`prometheus {}`, true, ""},
 		{`prometheus /foo`, true, ""},
+		{`prometheus a b c`, true, ""},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.input)