use string for keys instead of symbols, fix kiyoto/fluent-plugin-docker-metrics#2

Changed the record keys to string (they were symbols, except
  for the 'source' key). This change enables rewrite-tag-filter
  to use those keys in rewriterules
add gauge metric type, fix kiyoto/fluent-plugin-docker-metrics#4

set the type field as 'gauge' when it's not a counter
Merge branch 'add_gauge_metric_type'

Conflicts:
	lib/fluent/plugin/in_docker_metrics.rb