public long drainDurationMicros() {
        return drainDurationMicros;
    }
Returns {@link InitiateConnectionShutdown} event with custom drain duration in microseconds.
Negative values are a valid input - negative duration may be passed as a result of the time arithmetics,
in that case drain duration will be set to 0.
core/src/main/java/com/linecorp/armeria/internal/common/InitiateConnectionShutdown.java