Add support for intake API v2

[watson]

Needed to be released

• Implement core logic (PR feat: support APM Server intake API version 2 #465)
• Move truncation logic to apm-nodejs-http-client (PR refactor: remove built in payload truncation logic #498)
• Document apiRequestSize and apiRequestTime (PR docs: add api docs for apiRequestTime and apiRequestSize #499)
• Re-add support for DEBUG_PAYLOAD (PR chore: add payloadLogFile debug config option #500)
• Create benchmarks showing difference between old and new API
• Update HTTP client to use new endpoints (PR fix: use new v2 HTTP endpoint apm-nodejs-http-client#18)
• Update filtering API (PR feat: improved filtering function API #579)
• Add mock trace-id's (PR fix: update ids to correct v2 api format #584)
• Reenable schema tests using new JSON schemas from the v2 API (PR test: re-enable schema tests for the v2 api #585)
• Update APM Server json schema links to point to the master branch instead of v2
• Write upgrade docs from v1.x to v2.x (PR docs: add v2 upgrade guide #631)
• Stop sending data to the server if the server responds prematurely (PR fix: abort current request if server responds early apm-nodejs-http-client#32
• Ensure there's no remaining TODO's in the code related to v2
• Update doc links to APM Server documentation (in addFilter + metadata) (server issue: Add Intake v2 related documentation apm-server#1329, PR docs: update links for v2 #672)
• Ensure package.json dependencies point to npm and not temporary GitHub branches (PR chore: depend on elastic-apm-http-client@^6.0.0 #670)

Can be postponed to a minor or patch release

• Fix bug that wrongly counts uncompressed bytes of HTTP body instead of compressed (PR refactor: use new chopper transform feature to gzip apm-nodejs-http-client#25)
• Implement grace period algorithm in case of APM Server errors if it's offline (PR feat: add incremental back-off in case of APM Server issues apm-nodejs-http-client#17)
• Automatically run benchmarks (WIP)
• Parse returned json document in case of errors (PR fix: output APM Server error messages #671)
• Don't discard spans started after transaction ended

Nice to have

• Only flush if the error being sent is an uncaught exception

[roncohen]

great to have a list Thomas 👍
What about benchmarks?

[watson]

@roncohen good call. I've added it

[watson]

v2 is now done. I've moved all open tasks to their own separate issues