Use "Size" field to describe blobs over "Length"

After consideration, we've changed the main descriptor field name to for number
of bytes to "size" to match convention. While this may be a subjective
argument, commonly we refer to files by their "size" rather than their
"length". This will match other conventions, like `(FileInfo).Size()` and
methods on `io.SizeReaderAt`. Under more broad analysis, this argument doesn't
necessarily hold up. If anything, "size" is shorter than "length".

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/cache/suite.go

@@ -35,14 +35,14 @@ func checkBlobDescriptorCacheEmptyRepository(t *testing.T, ctx context.Context,
 
 	if err := cache.SetDescriptor(ctx, "", distribution.Descriptor{
 		Digest:    "sha384:abc",
-		Length:    10,
+		Size:      10,
 		MediaType: "application/octet-stream"}); err != digest.ErrDigestInvalidFormat {
 		t.Fatalf("expected error with invalid digest: %v", err)
 	}
 
 	if err := cache.SetDescriptor(ctx, "sha384:abc", distribution.Descriptor{
 		Digest:    "",
-		Length:    10,
+		Size:      10,
 		MediaType: "application/octet-stream"}); err == nil {
 		t.Fatalf("expected error setting value on invalid descriptor")
 	}
@@ -60,7 +60,7 @@ func checkBlobDescriptorCacheSetAndRead(t *testing.T, ctx context.Context, provi
 	localDigest := digest.Digest("sha384:abc")
 	expected := distribution.Descriptor{
 		Digest:    "sha256:abc",
-		Length:    10,
+		Size:      10,
 		MediaType: "application/octet-stream"}
 
 	cache, err := provider.RepositoryScoped("foo/bar")