Span status has no analogue in extension

[klibbbs]

The span data maintained by the extension (in opencensus_trace_span_t) has no field corresponding with the status property on OpenCensus\Trace\Span. This discrepancy is exposed when a trace is reported on shutdown, because OpenCensus\Trace\RequestHandler::addCommonRequestAttributes sets the outgoing HTTP response code on the root span.

The status field can only be set directly on an OpenCensus\Trace\Span object itself, unlike attributes, timeEvents, and links, which are set with methods on OpenCensus\Trace\Tracer. OpenCensus\Trace\Tracer\ExtensionTracer wraps extension built-ins for setting these other properties (e.g. opencensus_trace_add_attribute).

I think the idiomatic way to fix this would be to add a OpenCensus\Trace\Tracer::setStatus method (as well as the corresponding methods on the TracerInterface implementers) that would allow a new opencensus_trace_set_status built-in to be invoked in the same way.