Merge metrics tags before passing them to the handler

[hauleth]

Currently almost all metric handlers need to do something like:

maps:merge(CTags, maps:from_list(lists:zip(Tags, TagsV)))

Which is little bit PITA. What I am proposing is to do it for them in the oc_stats:export/0 and return data with above "merging" function already done. This should reduce hurdle in defining metrics handlers and should unbound them from the internal storage of data in the application.

[hauleth]

Alternatively use opencensus_service and provide list of tag names and list of tag values and skip CTags, just split them before and join it with Tags and TagsV beforehand.