Single Opamp with "Isolated" positive input pin or method to invert the feedback loop ? (SOT-23-5 or other package)
The input bias current is in the 10-100nA range. This means that the source would have to have an impedance with at least that. The LM358 does not need to be guarded, because it's input bias current does not support an application that would require a low leakage trace. 


Guards are used when the leakage through the PCB is high enough that it affects the signal, and typically is used in applications that have source impedance's below 10pA (at this point, the PCB becomes a conductor).

Edit:

This opamp will have poor results with a PH meter. Get one with the highest input bias current you can find, with the price that works for you. Single opamps in 8 pin packages usually work better for guarding as they have NC pins. Most op amps that have low input bias currents will also have pins that support guarding. Most opamps that have a low input bias current will also have layout guidelines for guarding.

More Edit:
If your running a PH meter, the guard trace would look like this:




Source: https://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf