Fix invalid log message format (#845)
Fix invalid log message format (#845)

Currently metrics starting log is not correctly formatted as:

```
$ kubectl  -n knative-eventing logs eventing-controller-756d56fc7-g64cd
  ...
{"level":"info" ..snip.. "caller":"controller/main.go:140","msg":"Starting metrics listener at %s:9090","knative.dev/controller":"controller"}
```

This patch makes a tiny change to fix it, which replaces `logger.Info`
with `logger.Infof`.
knative/eventing,knative/eventing