Allow metadata in register transaction

[lahsivjar]

The PR updates the /register/transaction endpoint to accept NDJSON. The /register/transaction endpoint can now accept metadata as the first event in NDJSON body and cache it for future calls. A complete list of changes introduced in the PR is as follows:

1. Change the Content-Type for the /register/transaction endpoint from application/vnd.elastic.apm.transaction+json to application/vnd.elastic.apm.transaction+ndjson.
2. Add support for deflate and gzip content encoding for the /register/transaction endpoint.
3. Allow metadata as the first event in the NDJSON body to the /register/transaction endpoint. The metadata is cached and used to send data to APM-Server. The cached metadata is NOT updated by the metadata sent via the intake endpoint.

Related Issue

Closes #352

[basepi]

The cached metadata is NOT updated by the metadata sent via the intake endpoint.

This is slightly concerning to me. We have a mechanism for adding "late-breaking" metadata in the python agent, for metadata that is only available at request time. For now, this should not be a problem -- the only late metadata we add is for lambda, and that metadata will be available when we send the partial transaction.

I just wanted to call it out as something we should explicitly state when we update the spec with partial transaction support.

[basepi]

👍 to stated changes, haven't reviewed the code.

[trentm]

The functionality sounds good. I won't have a chance to test this for a little while yet (hopefully this week).

[marclop]

is it OK to break backwards compatibility here? Is there a concern for using older agents? AFAICS, if older agents attempt to send data to this endpoint, we'll just reject it and lose the partial transaction information.

[lahsivjar]

This hasn't been implemented yet by any agent, in fact python agent is the first one to start implementing it so backward compatibility is not an issue at this point.

[marclop]

Code LGTM

[lahsivjar]

@trentm @basepi Did you get a chance to test this out? I have done some testing with go-agent and it seems to work as expected but wanted to make sure all requirements are appropriately covered before merging.

[trentm]

@lahsivjar I have not yet, but I think I may start looking at this tomorrow (as part of elastic/apm-agent-nodejs#3136).

[basepi]

I'm working on testing this today. I was putting it off because I haven't gotten around to building the extension from source yet. 😅

[basepi]

Tested, working great!