vendor: add buildkit

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/grpc-ecosystem/grpc-opentracing/go/otgrpc/shared.go

@@ -0,0 +1,42 @@
+package otgrpc
+
+import (
+	"strings"
+
+	opentracing "github.com/opentracing/opentracing-go"
+	"github.com/opentracing/opentracing-go/ext"
+	"google.golang.org/grpc/metadata"
+)
+
+var (
+	// Morally a const:
+	gRPCComponentTag = opentracing.Tag{string(ext.Component), "gRPC"}
+)
+
+// metadataReaderWriter satisfies both the opentracing.TextMapReader and
+// opentracing.TextMapWriter interfaces.
+type metadataReaderWriter struct {
+	metadata.MD
+}
+
+func (w metadataReaderWriter) Set(key, val string) {
+	// The GRPC HPACK implementation rejects any uppercase keys here.
+	//
+	// As such, since the HTTP_HEADERS format is case-insensitive anyway, we
+	// blindly lowercase the key (which is guaranteed to work in the
+	// Inject/Extract sense per the OpenTracing spec).
+	key = strings.ToLower(key)
+	w.MD[key] = append(w.MD[key], val)
+}
+
+func (w metadataReaderWriter) ForeachKey(handler func(key, val string) error) error {
+	for k, vals := range w.MD {
+		for _, v := range vals {
+			if err := handler(k, v); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}