this allows gelf logger to collect extra metadata from containers with
`--log-opt labels=label1,label2 --log-opt env=env1,env2`
Additional log field will be prefixed with `_` as per gelf protocol
https://www.graylog.org/resources/gelf/
Signed-off-by: Daniel Dao <dqminh@cloudflare.com>
Upstream-commit: 5794a0190d7505e998116f1321ae39b001d0e710
Component: engine