Arduino and Raspberry Pi - initiate I2C communication as slave
You need to use a separate signalling method to alert the Pi that a sensor value is ready to be collected.

This is usually done using an interrupt signal. Your Arduino changes the state of a GPIO pin which is connected to a GPIO pin (through suitable logic level translation as necessary) on the Pi. When the Pi sees that pin change state it requests the data from the Arduino.

It is most common to have the transition from HIGH to LOW as an interrupt signal (aka Active Low) since that allows a simple wired-OR arrangement (open drain GPIOs with a pullup resistor) to allow more than one device to raise the same interrupt.