middleware/metrics: allow multiple listeners

There was no inherent reason *not* to allow multiple listeners for the
monitoring data. Actually enforcing only one listener lead to more code
then just allowing multiple. It's probably not what you want; but
CoreDNS is happy to oblige.

Add middleware by directly linking it from caddy, i.e. without any code
changes. To be fair: this does not added a ServeHTTP, but does give
some nice features in the Corefile.

* Fix race on backend health status update
* Ensure test case is compatible on Go 1.7

Allow HTTP health check to be performed against a regular DNS upstream server.

TODO: Add tests.

Also don't default to a default proxy; keep it empty.
Normalize the zones, so that is the server block ones are used, it will
actually work.

* add proxy tcp

* add truncated for tcp to udp response

* move truncation to scrubbing

* add test that executes upstream over tcp

* middleware/proxy: some tweaks

rename force-tcp to force_tcp to be inline with the rest and use
a dnsOptions struct to put the options in to allow it to be extended.
Add some parse tests as well.

* Fix test and rename dnsOptions Options

According to the code, the default health check interval is 30 seconds,
not 10 as stated in the documentation.  (The alternative would be to
adjust the interval in the code.)

* core: add listening for other protocols

Allow CoreDNS to listen for TLS request coming over port 853. This can
be enabled with `tls://` in the config file.

Implement listening for grps:// as well.

a Corefile like:

~~~
. tls://.:1853 {
    whoami
    tls
}
~~~

Means we listen on 1853 for tls requests, the `tls` config item allows
configuration for TLS parameters. We *might* be tempted to use Caddy's
Let's Encrypt implementation here.

* Refactor coredns/grpc into CoreDNS

This makes gRPC a first class citizen in CoreDNS. Add defines as being
just another server.

* some cleanups

* unexport the servers

* Move protobuf dir

* Hook up TLS properly

* Fix test

* listen for TLS as well. README updates

* disable test, fix package

* fix test

* Fix tests

* Fix remaining test

* Some tests

* Make the test work

* Add grpc test from #580

* fix crash

* Fix tests

* Close conn

* README cleanups

* README

* link RFC

The deps target needs to be dependent on the generated middleware code,
or you end up having to do make twice.

We don't need to use the muxer in Go DNS s we only have one entry point
per server - and mux ourselves in ServeDNS. Also make ServeDNS take a
context, and make that the canonical way to call all middleware.

* Add edns0 code rewrite

* check arg count

* change `new`; set EDNS0 if request doesn't have it set

* change set to replace_or_append

* change to append_or_replace

* return error in new

* update documents

* fixt UT

* return error

* go fmt

* Rework for more general EDNS0 use

Also changed how rules are created and validated. Implements
EDNS0 NSID in addition to local.

* go fmt

* README updates, NSID tests and fixes

* gofmt -s -w

* Fix tests for rewrite syntax change

* Add tests, fix error message

* Review nits

* Missed on nit

* More tests, integration test, fix edns0 parse issue

* Fix README, use RewriteIgnored

* go fmt

In both etcd and k8s don't error log NXDOMAIN as this log spams the logs
for no good reason.

Fixes #568

Better long term solution is log rate limiting for both *log* and
*error*.

Set of small cleanups.

This fix updates the Makefile to add the `go lint` check
for the build. This fix also fixes several go lint issues.

NOTE: This fix does not enforce `go lint` (suggestion only).
This fix also ignores the `go lint` error:
```
middleware/middleware.go:72:1: context.Context should be the first parameter of a function
```
as it requires too many changes in API.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add CNCF Code of Conduct
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

* Fix reference to CNCF

https://cncf.io

This fixes a crash when we resolve (or try to) an external CNAME
when no proxy is set.

Add test as well.

Make the docker release part of the normal CoreDNS release

* checkpoint

* Pass context through ServeDNS, enable gRPC tracing

* Fix types and make tracer available to proxy. go fmt

* Fix imports

* Use the DoNotStartTrace option

* Change to SpanFilter from DoNotStartTrace

* Use new name (IncludeSpan)

* Final names

* Add tests; fix possible client/conn leaks in grpc

* go fmt

This fix adds the dependency of `go fmt` in `Makefile`
so that any new check in will have to properly fmt the source code
with `gofmt -s -w`.

* Gofmt all code

* fmt

This fix fixes incorrect link of the CodeCov in README.md.

* linter fixes

* Golint and format code

* fmt

This fix fixes import path from
`github.com/miekg/coredns`
->
`github.com/coredns/coredns`

This fix fixes badge in README.md so that it will correctly reflect
github.com/coredns/coredns

We now sit under coredns/coredns this means the logo can be in it's
repository.

* middleware/metrics: survive restart

Keep the handler running during restart. Stopping and starting the
handler results in "address in use" - sometimes, meaning the reload
will be flaky. In turn this behavior means any changes to the monitor
stanza are not picked up.

* remove resync

* Document fallthrough and fix *reverse*

While documenting the fallthrough behavior and testing it I noticed
the did not properly work. This PR does a tiny bit too much as it

- Documents fallthrough
- Fixes fallthrough in reverse
- Makes directives_generate complain on duplicate priorities
- Moved reverse *before* file in middleware.cfg
- Add a test that tests the reverse fallthrough behavior with a file
  backend

Fixes #515

* ....and fix the tests

Add additional section processing for MX and SRV records. Update the
tests.