all: fix plugin import ordering (#1717)

Got a bit messed up with stb lib "context" usage.

plugin/metrics/context.go

@@ -1,9 +1,9 @@
 package metrics
 
 import (
-	"github.com/coredns/coredns/plugin/metrics/vars"
-
 	"context"
+
+	"github.com/coredns/coredns/plugin/metrics/vars"
 )
 
 // WithServer returns the current server handling the request. It returns the

plugin/metrics/handler.go

@@ -1,14 +1,14 @@
 package metrics
 
 import (
+	"context"
+
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics/vars"
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/pkg/rcode"
 	"github.com/coredns/coredns/request"
 
-	"context"
-
 	"github.com/miekg/dns"
 )
 

plugin/metrics/metrics_test.go

@@ -1,6 +1,7 @@
 package metrics
 
 import (
+	"context"
 	"testing"
 
 	"github.com/coredns/coredns/plugin"
@@ -8,8 +9,6 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 
-	"context"
-
 	"github.com/miekg/dns"
 )
 

plugin/metrics/vars/report.go

@@ -1,13 +1,12 @@
 package vars
 
 import (
+	"context"
 	"time"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/request"
 
-	"context"
-
 	"github.com/miekg/dns"
 )