Objects do not propagate across parfor, need save/load methods

[duncanpo]

The following code would fail:

tr = opentelemetry.trace.getTracer("parfor_example");
sp = startSpan(tr, "main function");
...
parfor i = 1:8
% start a span for this iteration, using the tracer “tr” from main function
sp_i = startSpan(tr, "Iteration" + i);
...
end

Since tr is used both in and outside of the parfor block, parfor would copy and send it to the workers. Running this code would result in this error:

Error using . (line 63)
Unknown proxy ID 3

Error in opentelemetry.trace.Tracer/startSpan (line 125)
id = obj.Proxy.startSpan(spname, contextid, spankind, starttime, ...

This is because the global singleton proxymanager is not copied. In the workers, the proxies only exist in MATLAB but not in C++.

To make objects correctly propagate through to parfor blocks, they need to implement custom save/load methods which should also save/load their C++ counterparts.