Update documentation to clarify the default format for logs

[dduportal]

Signed-off-by: dduportal 1522731+dduportal@users.noreply.github.com

What does this PR do?

This PR clarifies the documentation page about logs:

• Adding comment to explicit that the default value is common and refers to the default CLF format
• Clarifies that customizing the fields requires to switch to the JSON format

Motivation

This PR follows up #3942 and is inspired by #3870 where the user was not aware about the mandatory format switch, which could have avoided wasting time searching for a non existent configuration.

Closes #4869.

More

• [ ] Added/updated tests
• Added/updated documentation

Additional Notes

[dduportal]

=> To be done: we need to check, there might be an issue classified as "bug/possible" around setting the format to the value common which does not work...

[dduportal]

@ldez it looks like that the value common is the right one (by default):

• https://github.com/containous/traefik/blob/v1.7/cmd/configuration.go#L195
• https://github.com/containous/traefik/blob/v1.7/middlewares/accesslog/logger.go#L29

Also, I tried to set accesslog.format (with CLI below, but also tried with traefik.toml) to common, and no error and the behavior is the same as only enabling accesslog alone

version: '3'

services:
  lb:
    image: traefik:v1.7.12
    restart: unless-stopped
    command: --api --docker --accesslog.format=common
    ports:
      - 80:80
      - 8080:8080
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    labels:
      - traefik.enable=false

  webapp:
    image: containous/whoami
    labels:
      - "traefik.frontend.rule=Host:localhost"

I'm removing the bot/no-merge and this PR is ready to review (unless I missed something of course!)

[ldez]

LGTM

[ldez]

LGTM

[SantoDE]

LGTM

[jbdoumenjou]

LGTM