add prefix to graphite tags

[skob]

if graphite tags are service or something like, it's just silently disappears

[jamtur01]

Can you give me some more information on what you're seeing here?

[skob]

if event from graphite come with tags from reserved keywords (like my.series;service=value1;host=value2 etc), then event become {:host "value2", :service "value1"}

in this PR I just add prefix , so event will be like {:host "original host", :service "original service", :tag_host "value2", :tag_service "value1"}