Comment 2 for bug 1790920

Effectively when using mongodb 3.2 you need to restart the service (see https://docs.mongodb.com/v3.2/tutorial/change-oplog-size/ ), in 3.6 it's no longer needed (see https://docs.mongodb.com/v3.6/tutorial/change-oplog-size/ ), so juju would need to check which version of mongodb is running and if it's >=3.6 accept the change, otherwise return an error indicating why it's not possible to change it

Methods to change the oplog size would be:

juju bootstrap --config mongodb-oplog-size=1234 # mongodb >= 3.2
juju model-config -m controller mongodb-oplog-size=1234 # mongodb >= 3.6