`ConsoleExporter` serializes raw julia objects causing performance issues

[krynju]

We've had a case of a single threaded service being overwhelmed by passing spans to a ConsoleExpoterer(devnull)
The process would hit 100% cpu usage for minutes at a time.
Switching to DummyExporter fixed the issue

Spans should ideally serialize into some simple human readable json/yaml format
Currently they serialize the whole struct and they show too many things