Log LFS smudge activity to gitaly_lfs_smudge.log

This commit improves the observabiilty of the Gitaly LFS smudge filter by writing JSON-formatted log messages into gitaly_lfs_smudge.log. For example:

{
  "content_length_bytes": 1010253,
  "correlation_id": "80dWci8lxB7",
  "duration_ms": 217,
  "level": "info",
  "method": "GET",
  "msg": "Finished HTTP request",
  "status": 200,
  "time": "2020-11-03T05:03:33Z",
  "url": "http://localhost:1234/api/v4/internal/lfs?oid=988f6456ac72429809eb61c699fc9830cca42bcbce96c229f038dc6206fd9571&gl_repository=project-8"
}