docs: Optional<String> writeFunctionTimer(FunctionTimer timer) {
          return Optional.of(writeDocument(timer, builder -> {
              builder.append(",\"count\":").append(timer.count());
              builder.append(",\"sum\" :").append(timer.totalTime(getBaseTimeUnit()));
              builder.append(",\"mean\":").append(timer.mean(getBaseTimeUnit()));
          }));
      }