Merge pull request #3 from rody/fix_issue_2

use string for keys, fix kiyoto/fluent-plugin-docker-metrics#2
add gauge metric type, fix kiyoto/fluent-plugin-docker-metrics#4

set the type field as 'gauge' when it's not a counter
Merge pull request #5 from rody/add_gauge_metric_type

add gauge metric type, fix kiyoto/fluent-plugin-docker-metrics#4
add more container info to record

Add the following fields to the record:
  . container_id
  . container_short_id
  . container_name
  . container_image
Merge branch 'rebase_original'