private static Optional<Instant> toInstantFromString(final String string) {
        if (string == null) {
            return Optional.empty();
        }

        try {
            return Optional.of(Instants.parseInstant(string));
        } catch (final NumberFormatException ex) {
            logger.warn("\"transaction_time\" is in an invalid format: '" + string + "'. "
                        + "The transaction time is set to the present.", ex);
            return Optional.empty();
        } catch (final IllegalStateException ex) {
            logger.error("Unexpected failure in parsing \"transaction_time\": '" + string + "'", ex);
            throw ex;
        }
    }
Provides access inside Embulk through a transactional session.

<p>It is internal implementation of {@link ExecSession} although some methods, such as {@code newPlugin}, are to be
removed from {@link ExecSession} soon during Embulk v0.10 when {@link ExecSession} moves to {@code embulk-spi} from
{@code embulk-core}. Even after the removal, they stay in this {@link ExecSessionInternal} to be called through
{@link ExecInternal}.
embulk-core/src/main/java/org/embulk/spi/ExecSessionInternal.java