global: move to context (#1699)

* global: move to context

Move from golang.org/x/net/context to std lib's context.

Change done with:

for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done

* drop from dns.pb.go as well

plugin/metrics/context.go

@@ -3,7 +3,7 @@ package metrics
 import (
 	"github.com/coredns/coredns/plugin/metrics/vars"
 
-	"golang.org/x/net/context"
+	"context"
 )
 
 // WithServer returns the current server handling the request. It returns the

plugin/metrics/handler.go

@@ -7,8 +7,9 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/rcode"
 	"github.com/coredns/coredns/request"
 
+	"context"
+
 	"github.com/miekg/dns"
-	"golang.org/x/net/context"
 )
 
 // ServeDNS implements the Handler interface.

plugin/metrics/metrics_test.go

@@ -8,8 +8,9 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 
+	"context"
+
 	"github.com/miekg/dns"
-	"golang.org/x/net/context"
 )
 
 func TestMetrics(t *testing.T) {

plugin/metrics/vars/report.go

@@ -6,8 +6,9 @@ import (
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
 
+	"context"
+
 	"github.com/miekg/dns"
-	"golang.org/x/net/context"
 )
 
 // Report reports the metrics data associcated with request.