Add parsing for nested json fields in the rules sampler

[ecobrien29]

Which problem is this PR solving?

Adds support for checking nested JSON when validating rules. The option can be enabled using the CheckNestedFields configuration option.

• Closes allow nested JSON fields for rules sampling config #413

Short description of the changes

• Adds a json parser module to allow retrieval of nested fields for the rules sampler. If the field is not found in the top level of the span, it calls the get function to check if it exists in the nested json.

[MikeGoldsmith]

I like the overall solution but I'm concerned with enabling JSON parsing of span data for all condition failures. This should be configuration driven to enable his behaviour.

[MikeGoldsmith]

I'm concerned this would impact users that does not want to pay the cost of attempting to JSON parse span data when trying to resolve rule conditions.

I think we should look into adding an opt-in configuration option to enable this behaviour. Default would be disabled.

[ecobrien29]

That makes sense. I can add an optional parameter in the rules config file to enable it (similar to the AddSampleRateKeyToTrace and UseTraceLength fields), unless you think there is a better place for it.

[ecobrien29]

I made the requested changes and tested with the refinery I have running with the nested fields parameter both enabled and disabled. The nested traces were correctly sampled when enabled and didn't affect normal sampling rules.

[vreynolds]

Thanks @ecobrien29! We've had a busy couple weeks, but we'll take another look as soon as we can!

[MikeGoldsmith]

Looks good, thank you for the follow-up and sorry for the delay in getting back to you.