Implement config singleton (#1268)

* Implement config singleton

* Pass token in grpc request headers

* Refactor BearerScheme

* Fix typo

grpc.go

@@ -18,6 +18,7 @@ import (
 	"github.com/micro/go-micro/v2/errors"
 	"github.com/micro/go-micro/v2/metadata"
 	"github.com/micro/go-micro/v2/registry"
+	"github.com/micro/go-micro/v2/util/config"
 
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
@@ -25,6 +26,10 @@ import (
 	gmetadata "google.golang.org/grpc/metadata"
 )
 
+var (
+	BearerScheme = "Bearer "
+)
+
 type grpcClient struct {
 	opts client.Options
 	pool *pool
@@ -128,6 +133,10 @@ func (g *grpcClient) call(ctx context.Context, node *registry.Node, req client.R
 	header["timeout"] = fmt.Sprintf("%d", opts.RequestTimeout)
 	// set the content type for the request
 	header["x-content-type"] = req.ContentType()
+	// set the authorization token if one is saved locally
+	if token, err := config.Get("token"); err == nil && len(token) > 0 {
+		header["authorization"] = BearerScheme + token
+	}
 
 	md := gmetadata.New(header)
 	ctx = gmetadata.NewOutgoingContext(ctx, md)