kubernetes source: Added key-delimiter() option.
Some metadata fields can contain .-s in their name. This does not work with syslog-ng-s macros, which
by default use . as a delimiter. The added key-delimiter() option changes this behavior by storing
the parsed metadata fields with a custom delimiter. In order to reach the fields, the accessor side has
to use the new delimiter format, e.g. --key-delimiter option in $(format-json).