Limit the length for all fields

All fields should be limited to KEYWORD_MAX_LENGTH, with the exception of the fields noted in the [long fields spec](https://github.com/elastic/apm/blob/main/specs/agents/field-limits.md#long_field_max_length-configuration), which should be limited to a length of 10000. Optionally, we can make the latter limit configurable, as stated in the spec.