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.

Start with the long fields spec at specs/agents/field-limits.md, especially the long_field_max_length configuration section, and identify how its exceptions apply to the agent's fields. Verify the expected limits for regular and long fields, then determine whether the optional configurability is in scope and test that all fields follow the selected limits.

Written by the indexing model from the issue text.