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/davecgh/go-spew/spew/testdata/dumpcgo.go

@@ -0,0 +1,82 @@
+// Copyright (c) 2013 Dave Collins <dave@davec.name>
+//
+// Permission to use, copy, modify, and distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+// NOTE: Due to the following build constraints, this file will only be compiled
+// when both cgo is supported and "-tags testcgo" is added to the go test
+// command line.  This code should really only be in the dumpcgo_test.go file,
+// but unfortunately Go will not allow cgo in test files, so this is a
+// workaround to allow cgo types to be tested.  This configuration is used
+// because spew itself does not require cgo to run even though it does handle
+// certain cgo types specially.  Rather than forcing all clients to require cgo
+// and an external C compiler just to run the tests, this scheme makes them
+// optional.
+// +build cgo,testcgo
+
+package testdata
+
+/*
+#include <stdint.h>
+typedef unsigned char custom_uchar_t;
+
+char            *ncp = 0;
+char            *cp = "test";
+char             ca[6] = {'t', 'e', 's', 't', '2', '\0'};
+unsigned char    uca[6] = {'t', 'e', 's', 't', '3', '\0'};
+signed char      sca[6] = {'t', 'e', 's', 't', '4', '\0'};
+uint8_t          ui8ta[6] = {'t', 'e', 's', 't', '5', '\0'};
+custom_uchar_t   tuca[6] = {'t', 'e', 's', 't', '6', '\0'};
+*/
+import "C"
+
+// GetCgoNullCharPointer returns a null char pointer via cgo.  This is only
+// used for tests.
+func GetCgoNullCharPointer() interface{} {
+	return C.ncp
+}
+
+// GetCgoCharPointer returns a char pointer via cgo.  This is only used for
+// tests.
+func GetCgoCharPointer() interface{} {
+	return C.cp
+}
+
+// GetCgoCharArray returns a char array via cgo and the array's len and cap.
+// This is only used for tests.
+func GetCgoCharArray() (interface{}, int, int) {
+	return C.ca, len(C.ca), cap(C.ca)
+}
+
+// GetCgoUnsignedCharArray returns an unsigned char array via cgo and the
+// array's len and cap.  This is only used for tests.
+func GetCgoUnsignedCharArray() (interface{}, int, int) {
+	return C.uca, len(C.uca), cap(C.uca)
+}
+
+// GetCgoSignedCharArray returns a signed char array via cgo and the array's len
+// and cap.  This is only used for tests.
+func GetCgoSignedCharArray() (interface{}, int, int) {
+	return C.sca, len(C.sca), cap(C.sca)
+}
+
+// GetCgoUint8tArray returns a uint8_t array via cgo and the array's len and
+// cap.  This is only used for tests.
+func GetCgoUint8tArray() (interface{}, int, int) {
+	return C.ui8ta, len(C.ui8ta), cap(C.ui8ta)
+}
+
+// GetCgoTypdefedUnsignedCharArray returns a typedefed unsigned char array via
+// cgo and the array's len and cap.  This is only used for tests.
+func GetCgoTypdefedUnsignedCharArray() (interface{}, int, int) {
+	return C.tuca, len(C.tuca), cap(C.tuca)
+}