Instant#now() obtains the current instant from the system clock. If the system clock returns a precision of only up to microseconds, the Instant#toString simply truncates the last three zeros from the nine digits. If the system on which you are running Java 17 (correto17) returns a precision of nanoseconds, you can truncate it to the precision of microseconds using Instant#truncatedTo(java.time.temporal.TemporalUnit).

public class Main {
    public static void main(String args[]) {
        Instant instant = Instant.now();
        Instant truncatedToMicros = instant.truncatedTo(ChronoUnit.MICROS);
        System.out.println(truncatedToMicros);
    }
}

Learn more about the modern Date-Time API from Trail: Date Time.