code: @Nullable
    public List<Span> getTrace(String traceId) {
      try {
        return storage.spanStore().getTrace(traceId).execute();
      } catch (IOException e) {
        throw Platform.get().assertionError("I/O exception in in-memory storage", e);
      }
    }
docs: Retrieves a trace by ID which zipkin server has received, or null if not present.