This converts log event into a logs API model simply by transmuting the type
wrapper and dropping all arrays, which are not supported by the API. We could
flatten out the keys, as this is what New Relic does internally, and we used to
do that, but the flattening iterator accessed through
`LogEvent::convert_to_fields` adds quotes to dotted fields names, which produces
broken attributes in New Relic, and nesting objects is actually a (slightly)
more efficient representation of the key names.