* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
* 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
Make the methods that handle Metrics all use pointer receivers to fix
sync.Once not being initialized.
Finish the setup_test to test for failures. And make the check for the
address more strict and return an error when it does not have a port
number.
Add a toplevel test that starts a CoreDNS server with metrics enabled
so we catch these errors in the future.
