plugin/metrics: Switch to using promhttp instead of deprecated Handler (#1312)

prometheus.Handler is deprecated according to the godoc for the package so
instead we're using promhttp.

Additionally, we are exposing the Registry that metrics is using so other
plugins that are not inside of coredns can read the registry. Otherwise, if
we kept using the Default one, there's no way to access that from outside
of the coredns repo since it is vendored.

vendor/github.com/go-openapi/jsonreference/.github/CONTRIBUTING.md

@@ -0,0 +1,117 @@
+## Contribution Guidelines
+
+### Pull requests are always welcome
+
+We are always thrilled to receive pull requests, and do our best to
+process them as fast as possible. Not sure if that typo is worth a pull
+request? Do it! We will appreciate it.
+
+If your pull request is not accepted on the first try, don't be
+discouraged! If there's a problem with the implementation, hopefully you
+received feedback on what to improve.
+
+We're trying very hard to keep go-swagger lean and focused. We don't want it
+to do everything for everybody. This means that we might decide against
+incorporating a new feature. However, there might be a way to implement
+that feature *on top of* go-swagger.
+
+
+### Conventions
+
+Fork the repo and make changes on your fork in a feature branch:
+
+- If it's a bugfix branch, name it XXX-something where XXX is the number of the
+  issue
+- If it's a feature branch, create an enhancement issue to announce your
+  intentions, and name it XXX-something where XXX is the number of the issue.
+
+Submit unit tests for your changes.  Go has a great test framework built in; use
+it! Take a look at existing tests for inspiration. Run the full test suite on
+your branch before submitting a pull request.
+
+Update the documentation when creating or modifying features. Test
+your documentation changes for clarity, concision, and correctness, as
+well as a clean documentation build. See ``docs/README.md`` for more
+information on building the docs and how docs get released.
+
+Write clean code. Universally formatted code promotes ease of writing, reading,
+and maintenance. Always run `gofmt -s -w file.go` on each changed file before
+committing your changes. Most editors have plugins that do this automatically.
+
+Pull requests descriptions should be as clear as possible and include a
+reference to all the issues that they address.
+
+Pull requests must not contain commits from other users or branches.
+
+Commit messages must start with a capitalized and short summary (max. 50
+chars) written in the imperative, followed by an optional, more detailed
+explanatory text which is separated from the summary by an empty line.
+
+Code review comments may be added to your pull request. Discuss, then make the
+suggested modifications and push additional commits to your feature branch. Be
+sure to post a comment after pushing. The new commits will show up in the pull
+request automatically, but the reviewers will not be notified unless you
+comment.
+
+Before the pull request is merged, make sure that you squash your commits into
+logical units of work using `git rebase -i` and `git push -f`. After every
+commit the test suite should be passing. Include documentation changes in the
+same commit so that a revert would remove all traces of the feature or fix.
+
+Commits that fix or close an issue should include a reference like `Closes #XXX`
+or `Fixes #XXX`, which will automatically close the issue when merged.
+
+### Sign your work
+
+The sign-off is a simple line at the end of the explanation for the
+patch, which certifies that you wrote it or otherwise have the right to
+pass it on as an open-source patch.  The rules are pretty simple: if you
+can certify the below (from
+[developercertificate.org](http://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <joe@gmail.com>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)
+
+You can add the sign off when creating the git commit via `git commit -s`.