fix(registry): prevent file descriptor leak in WriteToTextfile

In WriteToTextfile, if an error occurs before the explicit tmp.Close()
(e.g., Gather fails or MetricFamilyToText fails), the temporary file
descriptor is never closed. The deferred os.Remove(tmp.Name()) does not
close the file.

Explicitly close the file on all error paths that happen before the
final close, ensuring the descriptor is always released.

Additionally, this fixes the issue on Windows where an open file cannot
be deleted, preventing the deferred os.Remove from succeeding.

Fixes: 1d54dab ("Add WriteToTextfile test")
Found by PostgresPro.
Signed-off-by: Maksim Korotkov <m.korotkov@postgrespro.ru>

Author: ProjectMutilation

prometheus/registry.go

@@ -641,10 +641,12 @@ func WriteToTextfile(filename string, g Gatherer) error {
 
 	mfs, err := g.Gather()
 	if err != nil {
+		tmp.Close()
 		return err
 	}
 	for _, mf := range mfs {
 		if _, err := expfmt.MetricFamilyToText(tmp, mf); err != nil {
+			tmp.Close()
 			return err
 		}
 	}