Side Effects in Painless During Scripted Metric Aggregation

[jdconrad]

I want to discuss the behavior described here by @hendrikmuhs :

I have a design question: Painless lets me get the source via e.g. 
params['_source']. In the context of scripted metric you get a nasty side-
effect passing a value created from params['_source'] from one phase to the 
next. Because it works by reference, the content of a painless variable 
changes and you get wrong results. In order to work correctly you must 
create a copy by e.g. using new HashMap(params['_source']).

I wonder whether this is a bug. Full example: https://discuss.elastic.co/t/latest-full-document-in-transform/221435/4

Questions:

1. Is this a bug?
2. What behavior do we expect here?

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[stu-elastic]

Related: #52103

[stu-elastic]

We should parse params and emit read only maps and lists.

[rjernst]

Source fields should be accessed through the fields API now.