public T getValue() {
            return value;
        }
Return the last instant/value for the given MetricKey, or Optional.empty if there isn't one.

@param metricKey the key for which to calculate a getAndSet.
@param now the timestamp for the new value.
@param value the current value.
@return the timestamp of the previous entry and its value. If there
    isn't a previous entry, then this method returns {@link Optional#empty()}
clients/src/main/java/org/apache/kafka/common/telemetry/internals/LastValueTracker.java