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/blobserver.go

@@ -41,7 +41,7 @@ func (bs *blobServer) ServeBlob(ctx context.Context, w http.ResponseWriter, r *h
 		http.Redirect(w, r, redirectURL, http.StatusTemporaryRedirect)
 	case driver.ErrUnsupportedMethod:
 		// Fallback to serving the content directly.
-		br, err := newFileReader(ctx, bs.driver, path, desc.Length)
+		br, err := newFileReader(ctx, bs.driver, path, desc.Size)
 		if err != nil {
 			return err
 		}
@@ -61,7 +61,7 @@ func (bs *blobServer) ServeBlob(ctx context.Context, w http.ResponseWriter, r *h
 
 		if w.Header().Get("Content-Length") == "" {
 			// Set the content length if not already set.
-			w.Header().Set("Content-Length", fmt.Sprint(desc.Length))
+			w.Header().Set("Content-Length", fmt.Sprint(desc.Size))
 		}
 
 		http.ServeContent(w, r, desc.Digest.String(), time.Time{}, br)