You might want to look at MartinR’s energy diverter. Learn→PV Diversion→PLL Diverter
Not of course as an energy diverter, but the measurement is phase locked to the a.c. waveform. You need a voltage input for it to work. The principle is a sample close to the zero crossing is measured, and the amount it is away from zero adjusts the timing of the loop, so that you get an exact number of samples at exactly the same point on the waveform each time. You could have four samples per cycle (that’s the simplest concept), one would be at the positive-going crossing, the next at the positive peak, then the 3rd sample at the negative-going crossing and the final sample of the cycle at the negative peak. There’s a big snag here: remember, the peak current won’t be exactly ¼ cycle after the zero crossing of the voltage due to current lagging the voltage, and that’s because your pump will be inductive; and you can’t lock to the current because it goes away when the pump is switched off, and it takes the loop a second or two to lock when it starts up.
You need not measure every cycle. Depending on how stable everything is, you could lock to the zero crossing every 5th cycle say, then time the two peak-reading samples of current at an adjustable time later - the first nominally 4166 µs after the crossing but adjustable to find the peak, and the second exactly 8333 µs after that.
Despite you thinking emonLib blocks, in normal use it actually samples for only 200 ms every 10 s, so in fact the processor is largely free for 98% of the time.