HTTP body truncation truncates at wrong length

According to [the documentation](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#capture-body), HTTP bodies should be truncated at a fixed 2k bytes. With the new algorithm that truncates all strings at 1024 bytes, this unfortunately overwrites that. So not only are we truncating twice, the second time we're truncating at the wrong size.

I think the best solution is moving the body truncation logic to the HTTP client instead of having it in the agent like it is today.