feat(v2): add BuildHeaders to header

refs: googleapis/gapic-generator-go#1301

v2/header.go

@@ -32,6 +32,7 @@ package gax
 import (
 	"bytes"
 	"context"
+	"net/http"
 	"runtime"
 	"strings"
 	"unicode"
@@ -135,3 +136,15 @@ func InsertMetadata(ctx context.Context, mds ...metadata.MD) context.Context {
 	}
 	return metadata.NewOutgoingContext(ctx, out)
 }
+
+// BuildHeaders is for use by the Google Cloud Libraries only.
+//
+// BuildHeaders extracts metadata from the outgoing context, joins it with any
+// other given metadata, and converts them into a http.Header.
+func BuildHeaders(ctx context.Context, mds ...metadata.MD) http.Header {
+	if cmd, ok := metadata.FromOutgoingContext(ctx); ok {
+		mds = append(mds, cmd)
+	}
+	md := metadata.Join(mds...)
+	return http.Header(md)
+}

v2/header_test.go

@@ -31,6 +31,7 @@ package gax
 
 import (
 	"context"
+	"net/http"
 	"reflect"
 	"testing"
 
@@ -109,3 +110,19 @@ func TestInsertMetadata(t *testing.T) {
 		t.Errorf("InsertMetadata(ctx, %q) = %q, want %q", mds, got, want)
 	}
 }
+
+func TestBuildHeaders(t *testing.T) {
+	existingMd := metadata.Pairs("key_1", "val_1")
+	ctx := metadata.NewOutgoingContext(context.Background(), existingMd)
+	mds := []metadata.MD{
+		metadata.Pairs("key_2", "val_21"),
+		metadata.Pairs("key_2", "val_22"),
+	}
+
+	got := BuildHeaders(ctx, mds...)
+
+	want := http.Header{"key_1": []string{"val_1"}, "key_2": []string{"val_21", "val_22"}}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("BuildHeaders(ctx, %q) = %q, want %q", mds, got, want)
+	}
+}