testrand: allow for Metadata().JSON() calls

testrand.Metadata() is never used in any tests. One of the reasons for
that is probably because it misses an easy way to transform it into raw
bytes. This change adds a JSON method to allow for easy chaining, such
as

m: = testrand.Metadata().JSON()

Change-Id: I89e7638ce4dc33a951f3aca8db03a7392e3707ad

Author: amwolff

testrand/rand.go

@@ -5,6 +5,7 @@
 package testrand
 
 import (
+	"encoding/json"
 	"io"
 	"math/rand"
 
@@ -142,14 +143,28 @@ func BucketName() string {
 	return string(b)
 }
 
+// TestMetadata represents randomly generated metadata. Combined with
+// its Metadata constructor and JSON method, it allows for constructions
+// in tests such as `m := testrand.Metadata().JSON()`.
+type TestMetadata map[string]string
+
+// JSON encodes t to a slice of bytes.
+func (t TestMetadata) JSON() []byte {
+	b, err := json.Marshal(t)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
+
 // Metadata creates random metadata mostly conforming to the restrictions of S3:
 // https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html#object-metadata
 //
 // NOTE: This may not generate values that cover all valid values (for Storj or
 // S3). This is a best effort to cover most cases we believe our design
 // requires and will need to be revisited when a more explicit design spec is
 // created.
-func Metadata() map[string]string {
+func Metadata() TestMetadata {
 	const (
 		// The actual limit is 2KiB, but there are overheads to encoding and encryption.
 		max = 1 * 1024