Add json format support for Traefik logs

[m3co-code]

This PR adds the possibility to use JSON format for the traefik logs.

Also it aligns the configuration to the accessLogs and deprecates the traefikLogsFile option.

Sample log line:

{"level":"info","msg":"Traefik version dev built on I don't remember exactly","time":"2017-09-05T02:34:52+02:00"}

This fixes #780.

[juliens]

LGTM 👏

[m3co-code]

@juliens thanks for the review! I added one more commit in order to keep the colors when the logs are written to stdout :D :)

[nmengin]

Hello @marco-jantke .

Great PR!

I just have one remark to do :
I think it can be great to modify and/or add a test in the access_log_test.go file.

WDYT about this suggestion?

[m3co-code]

@nmengin it's not about access logs :-) I could, of course, add an integration test for the traefik log itself, but I am not sure on what to assert.. What I could is pick some lines and verify that they are valid JSON, without looking into the contents. Do you think this would help?

[nmengin]

@marco-jantke

Sorry for the misunderstood 😉

Do you think this would help?

IMHO, if there are tests, the code is easier to maintain. Even if, of course, these modifications are not about the main code part.

LGTM 🗞️

[timoreimann]

Just to be sure because we've touched the subject recently: does the handling of the deprecated vs. new configuration options work as expected?

Also, have we documented which version takes precedence?

[m3co-code]

This is certainly a good point Timo. I tested with all the different constellations with CLI and toml configuration. The behaviour is consistent and as expected. Given the new configuration traefikLog.filePath is specified, it will always be used. Only in case it is not specified the deprecated option will be used.

I now added a warning about the usage of the deprecated and documented at the deprecated option documentation, that the new option takes precedence.

To summarise shortly:

traefik --traefiklogsfile=./deprecated_log --traefiklog --traefiklog.filepath=./new_log --traefiklog.format="json" will write to ./new_log in JSON format.

traefik --traefiklogsfile=./deprecated_log --traefiklog --traefiklog.format="json" will write to ./deprecated_log in JSON format.

[timoreimann]

Thanks for double checking @marco-jantke. LGTM.