this allows jsonfile logger to collect extra metadata from containers with `--log-opt labels=label1,label2 --log-opt env=env1,env2`. Extra attributes are saved into `attrs` attributes for each log data. Signed-off-by: Daniel Dao <dqminh@cloudflare.com> Upstream-commit: 0083f6e984894b4d3697c1ae63547c07eea697af Component: engine