[Bug] Tags of observables in Alerts are not created when promoted

[jeromeleonard]

Problem Description

When an alerts contains observables with tags and is promoted as Case, tags are not seen in observables details nor in observables list.

[alanjones]

I believe this has already been fixed in 4.0.1.

#1499

[To-om]

@alanjones #1499 merge tags only if observable already exists. Here it is another problem.