server: make ConnectCallback take a context instead

This makes the WithConnectCallback option for the server take a context
as its input instead of the rpc metadata. To obtain the metadata, the
callback function can extract it from the context.

server.go

@@ -8,7 +8,6 @@ import (
 	"github.com/relab/gorums/ordering"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
-	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/status"
 	"google.golang.org/protobuf/reflect/protoreflect"
 )
@@ -65,8 +64,7 @@ func (s *orderingServer) NodeStream(srv ordering.Gorums_NodeStreamServer) error
 	ctx := srv.Context()
 
 	if s.opts.connectCallback != nil {
-		md, _ := metadata.FromIncomingContext(ctx)
-		s.opts.connectCallback(md)
+		s.opts.connectCallback(ctx)
 	}
 
 	go func() {
@@ -107,7 +105,7 @@ func (s *orderingServer) NodeStream(srv ordering.Gorums_NodeStreamServer) error
 type serverOptions struct {
 	buffer          uint
 	grpcOpts        []grpc.ServerOption
-	connectCallback func(metadata.MD)
+	connectCallback func(context.Context)
 }
 
 // ServerOption is used to change settings for the GorumsServer
@@ -130,10 +128,9 @@ func WithGRPCServerOptions(opts ...grpc.ServerOption) ServerOption {
 
 // WithConnectCallback registers a callback function that will be called by the server
 // whenever a node connects or reconnects to the server. This allows access to the node's
-// connection metadata, which is passed to the callback function.
-//
-// NOTE: if there is no metadata present, the metadata parameter will be nil.
-func WithConnectCallback(callback func(metadata.MD)) ServerOption {
+// stream context, which is passed to the callback function. The stream context can be
+// used to extract the metadata and peer information, if available.
+func WithConnectCallback(callback func(context.Context)) ServerOption {
 	return func(so *serverOptions) {
 		so.connectCallback = callback
 	}

server_test.go

@@ -1,6 +1,7 @@
 package gorums_test
 
 import (
+	"context"
 	"net"
 	"testing"
 	"time"
@@ -15,7 +16,11 @@ func TestServerCallback(t *testing.T) {
 	var message string
 	signal := make(chan struct{})
 
-	srv := gorums.NewServer(gorums.WithConnectCallback(func(m metadata.MD) {
+	srv := gorums.NewServer(gorums.WithConnectCallback(func(ctx context.Context) {
+		m, ok := metadata.FromIncomingContext(ctx)
+		if !ok {
+			return
+		}
 		message = m.Get("message")[0]
 		signal <- struct{}{}
 	}))