feat: impl contextPropagationOnly config var; change disableSend behaviour

[trentm]

Implement contextPropagationOnly per spec (it does what disableSend
did before this change). Change disableSend to just disable comms
with APM server, but otherwise not attempt to reduce work.
https://github.com/elastic/apm/blob/master/specs/agents/transport.md#context_propagation_only-configuration

Closes: #2393

Checklist

• Implement code
• Add tests
• Update TypeScript typings
• Update documentation
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

[trentm]

@mshustov As mentioned in the issue comment my hope is that if there is a Node.js APM agent release with this PR out before elastic/kibana#112973 and elastic/kibana#102704 go in, that the Kibana issue and pull can be updated to use the config var name contextPropagationOnly rather than disableSend.

The reason for the config var name change is that disableSend was implemented in the Node.js APM agent when it looked like the spec PR was going to settle on this name. But then it turned out the config var name disableSend is used in other language APM agents (Python and Ruby) to mean something slightly different, so for consistency the functionality Kibana wants here is now called contextPropagationOnly.

Is this acceptable to you?

(If there is a mix-up and Kibana uses disableSend with an APM agent including this PR change, then the only down side will be that the APM agent does some extra undesired processing -- like collecting error stacktraces and collecting metrics.)

[apmmachine]

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• Start Time: 2021-11-02T16:04:52.008+0000

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[astorm]

Approving. Looks to do the thing it says it does. I see both disableSend and contextPropagationOnly will generate a NoOp transport, and contextPropagationOnly has a few extra guard clauses (replacing the old disableSend clauses) to prevent needed processing time. Config boiler plate looks to be setup correctly as well.

[mshustov]

@trentm sorry for the late response, I was vacationing. The proposed solution makes sense to me. elastic/kibana#112973 and elastic/kibana#102704 aren't going to be released in v8.0 anyway.

When are you going to release a client with these changes?
We can merge elastic/kibana#112973 and switch from disableSend to contextPropagationOnly as soon as the new version is available.

[trentm]

When are you going to release a client with these changes?

Either this week, or early next week. I'll ping on those two issues when we have a release with this in it. Thanks!

[trentm]

@mshustov FYI: elastic-apm-node@3.24.0 is now published which includes this change.