code: public static Span currentSpan(Object spanProvider)
      {
          SpanProvider sp = asFacet(SpanProvider.class, spanProvider);
          Span span = (sp == null ? null : sp.currentSpan());
          return span;
      }
docs: Attempts to get a {@link Span} from the given object, if it
  supports {@link SpanProvider#currentSpan()}.
  
  @param spanProvider may be null.
  @return null if there's not a current span.