Improve docker input plugin

[titilambert]

Hello !
This commit adds docker info data in returned metrics and improve test coverage to more than 70%

[sparrc]

thanks @titilambert, can you update the README with the new stats?

[titilambert]

Done !

[sparrc]

the way the metrics are formatted seems a little inconsistent to me, what about like this?

- docker
    - n_used_file_descriptors
    - n_cpus
    - n_containers
    - n_images
    - n_goroutines
    - n_listener_events
    - memory_total
    - pool_blocksize
- docker_data
    - available
    - total
    - used
- docker_metadata
    - available
    - total
    - used

[titilambert]

@sparrc you're right ! Changes done !

[tripledes]

@titilambert hey, looking at the diff just spotted a typo:

docker (memeory_total)

Nice job btw!

[titilambert]

@tripledes Thanks ! Fixed !

[sparrc]

change these to a single call to AddFields():

fields := map[string]interface{}{
  "n_cpus": ...
  "n_containers": ...
  ...
}
acc.AddFields("docker", fields, ...)

[titilambert]

Done ! I didn't merge memory_total because there is unit tag with.