It seems that I can connect to mongo this way:
sudo mongo mongodb://%2Fvar%2Fsnap%2Fwekan%2Fx1%2Fshare%2Fmongodb-27017.sock
And this could be correct unix socket setting:
sudo snap set wekan mongodb-bind-unix-socket='share'
I’m still trying to figure out how to get it to work, for example with unix socket.