plugin/metadata: some cleanups (#1906)

* plugin/metadata: some cleanups

Name to provider.go as that's what being defined right now in the file.
Use request.Request because that's done in variables.go anyway. Name the
main storage M, because there is no further meaning behind.

Remove superfluous methods

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix test

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/variables/variables_test.go

@@ -5,6 +5,8 @@ import (
 	"testing"
 
 	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
 	"github.com/miekg/dns"
 )
 
@@ -63,8 +65,9 @@ func TestGetValue(t *testing.T) {
 		m := new(dns.Msg)
 		m.SetQuestion("example.com.", dns.TypeA)
 		m.Question[0].Qclass = dns.ClassINET
+		state := request.Request{W: &test.ResponseWriter{}, Req: m}
 
-		value, err := GetValue(tc.varName, &test.ResponseWriter{}, m)
+		value, err := GetValue(state, tc.varName)
 
 		if tc.shouldErr && err == nil {
 			t.Errorf("Test %d: Expected error, but didn't recieve", i)