public CountersStats plus(CountersStats toAdd) {
        CountersStats result = new CountersStats();
        result.messagesSent += this.messagesSent;
        result.messagesReceived += this.messagesReceived;
        result.messageSendErrors += this.messageSendErrors;

        result.messagesSent += toAdd.messagesSent;
        result.messagesReceived += toAdd.messagesReceived;
        result.messageSendErrors += toAdd.messageSendErrors;
        return result;
    }
@Test
    void zeroPlus() {
        CountersStats zero = new CountersStats();
        CountersStats two = new CountersStats();
        two.messagesSent = 2;
        two.messageSendErrors = 20;
        two.messagesReceived = 200;

        CountersStats result = zero.plus(two);
        assertThat(result)
                .satisfies(
                        r -> {
                            assertThat(r.messagesSent).isEqualTo(2);
                            assertThat(r.messageSendErrors).isEqualTo(20);
                            assertThat(r.messagesReceived).isEqualTo(200);
                        });
    }