JEXL. Include previous values of atributes in expression context

[fgalan]

Is your feature request related to a problem / use case? Please describe.

Include the previous values of attributes in the context available for JEXL expression evaluation

Describe the solution you'd like

The current notification processing logic already know the previous values of attributes (I don't remember exactly how, but this is mechanism thanks to the previousValue metadata works).

It would be great to have this previous values at hand for JEXL evaluation, e.g. it would be very easy to calculate an increment this way: ${prevValue.A - A}.

Eg:

{
  "A": 2,
  "B": 3,
  "C": 4,
  "_prevValue": {
    "A": 1,
    "B": 3
  }
}

Describe alternatives you've considered

Another way could be this:

{
  "A.previous": 1,
  "A": 2,
  "B.previous": 3,
  "B": 3,
  "C": 4
  }
}

but it seems more problematic ("A.previous": 1 syntax collides with "A": { "previous": 1}.

However, take into account the existing implementation for previousValue metadata to pick the best alternative.

Describe why you need this feature

• To approach a new use case.
• To improve or simplify an scenario.

[fgalan]

Closed along with #4594