exporter classes should be handle classes

[duncanpo]

Some exporter classes (e.g. OtlpHttpSpanExporter) are currently implemented as value classes. However they contain handle properties which should not be shallow-copied. This can result in wrong and confusing behaviors.

import opentelemetry.sdk.trace.*
import opentelemetry.exporters.otlp.OtlpHttpSpanExporter
exp = OtlpHttpSpanExporter;                               % create an exporter
exp1 = exp;                                                           % make a copy
exp1.Endpoint = "bogus";                                    % mess up the endpoint of the copy
proc = SimpleSpanProcessor(exp);                      % use the original that supposedly be still using default settings
tp = TracerProvider(proc);                                    
tr = getTracer(tp, "foo");
sp = startSpan(tr, "bar");
endSpan(sp);

The above code will not export the span to the default collector location, because exp and exp1 still share the same backend even though they appear to be independent copies.