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