allow nested JSON fields for rules sampling config

[ecobrien29]

Is your feature request related to a problem? Please describe.
Yes, our logs have nested JSON fields that we would like to use for sampling decisions. The current code looks like it does not allow this functionality.

Describe the solution you'd like
I would like to modify the code here to also use a get function if the value isn't found in order to have it read nested JSON, but also retain the original functionality for fields with a name including a '.'

Describe alternatives you've considered

Additional context

[MikeGoldsmith]

Thanks for raising this @ecobrien29. We will take a look and add to our backlog for prioritisation.

We're also very open to community contributions, so if you feel so inclined we'd welcome a PR 👍🏻