The other answers are very good, but I want to elaborate on how micros() works. It always reads the current hardware timer (possibly TCNT0) which is constantly being updated by the hardware (in fact, every 4 µs because of the prescaler of 64). It then adds in the Timer 0 overflow count, which is updated by a timer overflow interrupt (multiplied by 256).

Thus, even inside an ISR, you can rely on micros() updating. However if you wait too long then you miss the overflow update, and then the result returned will go down (i.e you will get 253, 254, 255, 0, 1, 2, 3 etc.)

This is micros() - slightly simplified to remove defines for other processors:

unsigned long micros() {
    unsigned long m;
    uint8_t oldSREG = SREG, t;
    cli();
    m = timer0_overflow_count;
    t = TCNT0;
    if ((TIFR0 & _BV(TOV0)) && (t < 255))
        m++;
    SREG = oldSREG;
    return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

The code above allows for the overflow (it checks the TOV0 bit) so it can cope with the overflow while interrupts are off but only once - there is no provision for handling two overflows.

TLDR;

• Don't do delays inside an ISR
• If you must do them, you can time then with micros() but not millis(). Also delayMicroseconds() is a possibility.
• Don't delay more than 500 µs or so, or you'll miss a timer overflow.
• Even short delays may cause you to you miss incoming serial data (at 115200 baud you will get a new character every 87 µs).

If the timer 0 has overflowed (TIFR0 & _BV(TOV0)) and there is at least a count of 1 in the timer count register (t & 255) then increment the local copy of the overflow count.

That overflow count is used later on to add an upper 8 bits to what is basically an 8 bit counter to turn it into a 16 bit one ((m << 8) + t).