@Nullable public Span currentSpan() {
    TraceContext context = currentTraceContext.get();
    if (context == null) return null;
    // Returns a lazy span to reduce overhead when tracer.currentSpan() is invoked just to see if
    // one exists, or when the result is never used.
    return new LazySpan(this, context);
  }
@Test void currentSpan_defaultsToNull() {
    assertThat(tracer.currentSpan()).isNull();
  }