GenericParser should be resilient to bad fields

[jaffee]

The generic parser currently returns an error any time it fails to parse a field's value, and stops processing for that record. In reality, there are lots of innocuous reasons why a certain field might fail to parse, and it doesn't indicate that the entire record is suspect.

We should have stat/log options for notifying and counting which fields are failing, why, and how often, but we should make a best effort to parse any record and return some data.

I don't think we need to go crazy with configurability, some count stats that use the field path and encapsulate what the error is (e.g. null value, unsupported type, etc.). pdk/ingest.go has an example of a simple stats interface, that we should probably extend throughout the codebase so that it can be configured at the top level.