Well, that's not valid XML, but assuming it really ends with </metric-datas> then it can be turned into XML and split using
mutate { gsub => [ "message", "< ", "<" ] }
mutate { gsub => [ "message", "/ ", "/" ] }
xml { source => "message" store_xml => true target => "theXML" force_array => false }
split { field => "[theXML][metric-data]" }
If you select part of your post in the edit window and click on the </> button in the toolbar it may help with your formatting.