Added support for JSON based logging

[mvisonneau]

This PR enable support for getting JSON based logs out of the binaries.

It can be set using either

# ciliumd.yaml
log-opt:
  format: json

or --log-opt format=json

There is a slight issue related to some log lines that get written before the logger gets actually configured. I reckon it must also be the case when log-driver is set to syslog or the level is different than the default though.

Added support for logging in JSON format

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[coveralls]

Coverage increased (+0.007%) to 44.459% when pulling b40fad1 on mvisonneau:logging/json into 9f32546 on cilium:master.

[joestringer]

Thanks for the contribution! I have a couple of minor comments below on the code.

[aanm]

What's the rational behind this removal?

[mvisonneau]

my former comment on this got removed, my bad. I assumed this statement was not actually used given that the logrus.Formatter is configured with an hardcoded value for disabling the timestamp 🤔 DisableTimestamp = true

[aanm]

Well that explains stuff... and I thought it was because of the detection of os.Getenv("INITSYSTEM"). @borkmann you usually run cilium-agent standalone. Did you ever miss the lack of timestamps?

[borkmann]

Nope, not yet so far. ;)

[aanm]

Well that explains stuff... and I thought it was because of the detection of os.Getenv("INITSYSTEM"). @borkmann you usually run cilium-agent standalone. Did you ever miss the lack of timestamps?

[aanm]

test-me-please