Groupby multiple columns and calculate yearly and monthly percentage changes in R

Question

Given a dataframe df as follows:

df <- structure(list(city = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,