Change behaviour of ELASTIC_APM_INSTRUMENT config

[alvarolobato]

Add ELASTIC_APM_INSTRUMENT for disabling instrumentation modules. This should not affect OpenTracing or other custom instrumentation.

See elastic/apm#49

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Update @mikker: Currently blocked by elastic/apm#92 which will probably move this and a few other things around.

[mikker]

1. We already have an option called instrument but it means something else. Right now it switches all transactions on/off (and thereby also spans). Keeping error tracking.

2. Changing the behaviour of this would be a breaking change. If users want the functionality to match the new definition of this, they can disable most auto-instrumentation with config.disabled_instrumentations = config.available_instrumentations.

3. If the option is to be available via CentralConfig we also need to change the spies. Disabling one right now means not loading its Spy, ie. file. Every spy would need to have a hot-swappable disabled flag like for example the net/http one has. That way we'll load all files, then disable/enable the ones the user has chosen.

4. We also don't have a way to distinguish between manually started transactions and spans and agent-defined ones. I suppose we'd need to add some sort of flag to the start_X methods to distinguish, eg. ElasticAPM.start_span 'test', internal: true or auto: true or from_agent: true. All of them feel a bit weird but I do not want to put the responsibility of adding a flag on the user.

@estolfo, @axw wdyt?

[estolfo]

How does this relate to elastic/apm#92 ? It is a completely separate issue?

I would also bring up these important points in the agent issue, as Thomas said something similar to (4) regarding the node agent.

[mikker]

This issue was created before that discussion but you're right that it's deeply affected by it. I think we should sit on this and maybe resolve it along with that other issue instead which is still (sigh) being debated. Agree?

[estolfo]

Yes, this seems related to the other issue and they should be resolved/decided together.

[mikker]

Closing as instrument will be replaced by recording.