upsert
Inserts or updates a key-value pair in the ObservableMutableMap. If the key already exists, its value is updated using the provided transformation function. If the key does not exist, a new key-value pair is added with the value derived from the transformation function.
Parameters
key
The key to be added or updated in the map.
value
A function that computes the new value based on the current value (or null if the key does not exist).