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
Merge pull request #3 from rody/fix_issue_2

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