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/PuerkitoBio/purell/benchmarks/v0.1.0

@@ -0,0 +1,9 @@
+PASS
+BenchmarkSafe	  500000	      6131 ns/op
+BenchmarkUsuallySafe	  200000	      7864 ns/op
+BenchmarkUnsafe	  100000	     28560 ns/op
+BenchmarkAllDWORD	   50000	     38722 ns/op
+BenchmarkAllOctal	   50000	     40941 ns/op
+BenchmarkAllHex	   50000	     44063 ns/op
+BenchmarkAllCombined	   50000	     33613 ns/op
+ok  	github.com/PuerkitoBio/purell	17.404s