Add long_field_max_length configuration option

[trentm]

This is a follow up to the discussion issue for a common config var for some of the longer fields that (a) aren't limited in length by the APM server intake API schema and (b) have some current agent-specific issues open to be made configurable.

Rendered here: https://github.com/trentm/apm/blob/long_field_max_length/specs/agents/field-limits.md

Some reviewer notes:

• The user issues that led to this config option were for {transaction,error}.context.request.body and span.context.db.statement only. I added some other potential candidate fields for this config var {transaction,span,error}.context.message.body, error.exception.message, error.log.message. I have no strong preference on whether to include those other fields now, later or never. There are here for discussion. Thoughts? (The Node.js APM agent already has the errorMessageMaxLength config var for the latter two.)
• I'm ambivalent on "Central config: true" for this. It is certainly feasible, but are there reasons we'd consider avoiding adding rarely used config vars to the central config UI?

[apmmachine]

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• Start Time: 2021-08-25T15:30:36.880+0000

• Duration: 5 min 48 sec

• Commit: 2deecbe

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[trentm]

If the author of the PR deems the impact on product as negligible, approval by PM can be skipped.

I'm skipping PM approval.