lib/truncate and messaging context

The Agent will soon support [its first messaging spans](https://github.com/elastic/apm-agent-nodejs/pull/2013/files#diff-893516fb8ebf3304a7e9b5e27c0cc116846898af68e9b1aa01f2a45fe66dc5d4R652), which [will include messaging context fields](https://github.com/elastic/apm/blob/master/specs/agents/tracing-instrumentation-messaging.md#context-fields)

We should determine if these new fields need [special truncation rules in `contextLength`](https://github.com/elastic/apm-nodejs-http-client/blob/81771764dcb0d1e267c1a2b8e8013417fb9f9bb3/lib/truncate.js#L280), or if the default of `opts.truncateStringsAt` is sufficient.