[Examples] Memory ownership issues in InMemorySpanExporter usage example.

[marcalff]

In example multi_processor/main.cc, the code:

Builds a InMemorySpanExporter object

  InMemorySpanExporter *memory_span_exporter = new InMemorySpanExporter();
  auto exporter2 = std::unique_ptr<trace_sdk::SpanExporter>(memory_span_exporter);

Transfers memory ownership to a SpanProcessor

  auto processor2 = std::unique_ptr<trace_sdk::SpanProcessor>(
      new trace_sdk::SimpleSpanProcessor(std::move(exporter2)));

Use the underlying InMemorySpanExporter object directly.

  InMemorySpanExporter *memory_span_exporter = initTracer();

  foo_library();
  auto memory_spans = memory_span_exporter->GetData()->GetSpans();

This usage of InMemorySpanExporter is unsafe, as the exporter pointer is not owned, and the exporter could go away for example when installing a different global tracer provider (if I understand correctly).

Suggested fix:

initTracer() should return the underlying InMemorySpanData instead, which is safe to do because the data is held by a shared pointer.
Even if the exporter goes away, the data will still be accessible.

[marcalff]

Please assign to me.
Fixed by #1471