diff --git a/vendor/google.golang.org/grpc/metadata/metadata.go b/vendor/google.golang.org/grpc/metadata/metadata.go
new file mode 100644
index 0000000..cf6d1b9
--- /dev/null
+++ b/vendor/google.golang.org/grpc/metadata/metadata.go
@@ -0,0 +1,209 @@
+/*
+ *
+ * Copyright 2014 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package metadata define the structure of the metadata supported by gRPC library.
+// Please refer to https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md
+// for more information about custom-metadata.
+package metadata // import "google.golang.org/grpc/metadata"
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// DecodeKeyValue returns k, v, nil.
+//
+// Deprecated: use k and v directly instead.
+func DecodeKeyValue(k, v string) (string, string, error) {
+	return k, v, nil
+}
+
+// MD is a mapping from metadata keys to values. Users should use the following
+// two convenience functions New and Pairs to generate MD.
+type MD map[string][]string
+
+// New creates an MD from a given key-value map.
+//
+// Only the following ASCII characters are allowed in keys:
+//  - digits: 0-9
+//  - uppercase letters: A-Z (normalized to lower)
+//  - lowercase letters: a-z
+//  - special characters: -_.
+// Uppercase letters are automatically converted to lowercase.
+//
+// Keys beginning with "grpc-" are reserved for grpc-internal use only and may
+// result in errors if set in metadata.
+func New(m map[string]string) MD {
+	md := MD{}
+	for k, val := range m {
+		key := strings.ToLower(k)
+		md[key] = append(md[key], val)
+	}
+	return md
+}
+
+// Pairs returns an MD formed by the mapping of key, value ...
+// Pairs panics if len(kv) is odd.
+//
+// Only the following ASCII characters are allowed in keys:
+//  - digits: 0-9
+//  - uppercase letters: A-Z (normalized to lower)
+//  - lowercase letters: a-z
+//  - special characters: -_.
+// Uppercase letters are automatically converted to lowercase.
+//
+// Keys beginning with "grpc-" are reserved for grpc-internal use only and may
+// result in errors if set in metadata.
+func Pairs(kv ...string) MD {
+	if len(kv)%2 == 1 {
+		panic(fmt.Sprintf("metadata: Pairs got the odd number of input pairs for metadata: %d", len(kv)))
+	}
+	md := MD{}
+	var key string
+	for i, s := range kv {
+		if i%2 == 0 {
+			key = strings.ToLower(s)
+			continue
+		}
+		md[key] = append(md[key], s)
+	}
+	return md
+}
+
+// Len returns the number of items in md.
+func (md MD) Len() int {
+	return len(md)
+}
+
+// Copy returns a copy of md.
+func (md MD) Copy() MD {
+	return Join(md)
+}
+
+// Get obtains the values for a given key.
+func (md MD) Get(k string) []string {
+	k = strings.ToLower(k)
+	return md[k]
+}
+
+// Set sets the value of a given key with a slice of values.
+func (md MD) Set(k string, vals ...string) {
+	if len(vals) == 0 {
+		return
+	}
+	k = strings.ToLower(k)
+	md[k] = vals
+}
+
+// Append adds the values to key k, not overwriting what was already stored at that key.
+func (md MD) Append(k string, vals ...string) {
+	if len(vals) == 0 {
+		return
+	}
+	k = strings.ToLower(k)
+	md[k] = append(md[k], vals...)
+}
+
+// Join joins any number of mds into a single MD.
+// The order of values for each key is determined by the order in which
+// the mds containing those values are presented to Join.
+func Join(mds ...MD) MD {
+	out := MD{}
+	for _, md := range mds {
+		for k, v := range md {
+			out[k] = append(out[k], v...)
+		}
+	}
+	return out
+}
+
+type mdIncomingKey struct{}
+type mdOutgoingKey struct{}
+
+// NewIncomingContext creates a new context with incoming md attached.
+func NewIncomingContext(ctx context.Context, md MD) context.Context {
+	return context.WithValue(ctx, mdIncomingKey{}, md)
+}
+
+// NewOutgoingContext creates a new context with outgoing md attached. If used
+// in conjunction with AppendToOutgoingContext, NewOutgoingContext will
+// overwrite any previously-appended metadata.
+func NewOutgoingContext(ctx context.Context, md MD) context.Context {
+	return context.WithValue(ctx, mdOutgoingKey{}, rawMD{md: md})
+}
+
+// AppendToOutgoingContext returns a new context with the provided kv merged
+// with any existing metadata in the context. Please refer to the
+// documentation of Pairs for a description of kv.
+func AppendToOutgoingContext(ctx context.Context, kv ...string) context.Context {
+	if len(kv)%2 == 1 {
+		panic(fmt.Sprintf("metadata: AppendToOutgoingContext got an odd number of input pairs for metadata: %d", len(kv)))
+	}
+	md, _ := ctx.Value(mdOutgoingKey{}).(rawMD)
+	added := make([][]string, len(md.added)+1)
+	copy(added, md.added)
+	added[len(added)-1] = make([]string, len(kv))
+	copy(added[len(added)-1], kv)
+	return context.WithValue(ctx, mdOutgoingKey{}, rawMD{md: md.md, added: added})
+}
+
+// FromIncomingContext returns the incoming metadata in ctx if it exists.  The
+// returned MD should not be modified. Writing to it may cause races.
+// Modification should be made to copies of the returned MD.
+func FromIncomingContext(ctx context.Context) (md MD, ok bool) {
+	md, ok = ctx.Value(mdIncomingKey{}).(MD)
+	return
+}
+
+// FromOutgoingContextRaw returns the un-merged, intermediary contents
+// of rawMD. Remember to perform strings.ToLower on the keys. The returned
+// MD should not be modified. Writing to it may cause races. Modification
+// should be made to copies of the returned MD.
+//
+// This is intended for gRPC-internal use ONLY.
+func FromOutgoingContextRaw(ctx context.Context) (MD, [][]string, bool) {
+	raw, ok := ctx.Value(mdOutgoingKey{}).(rawMD)
+	if !ok {
+		return nil, nil, false
+	}
+
+	return raw.md, raw.added, true
+}
+
+// FromOutgoingContext returns the outgoing metadata in ctx if it exists.  The
+// returned MD should not be modified. Writing to it may cause races.
+// Modification should be made to copies of the returned MD.
+func FromOutgoingContext(ctx context.Context) (MD, bool) {
+	raw, ok := ctx.Value(mdOutgoingKey{}).(rawMD)
+	if !ok {
+		return nil, false
+	}
+
+	mds := make([]MD, 0, len(raw.added)+1)
+	mds = append(mds, raw.md)
+	for _, vv := range raw.added {
+		mds = append(mds, Pairs(vv...))
+	}
+	return Join(mds...), ok
+}
+
+type rawMD struct {
+	md    MD
+	added [][]string
+}