Skip Async-http instrumentation installation when Traces::Backend::NewRelic is defined

[tannalynn]

If the traces-backend-newrelic instrumentation is being used, then skip async-http instrumentation installation since this gem provides instrumentation.
https://github.com/newrelic/traces-backend-newrelic

[kaylareopelle]

Nice! I have a few questions, and the answers can all be no 🙂

• Should we add a changelog entry for this? It might be as close as we get to mentioning the new gem in the main agent's release notes.
• Would it be helpful to have a comment around line 15 describing where Traces::Backend::NewRelic is defined? I'm worried someone might have trouble finding it since it's not part of the async-http library.
• Do we want to add a test for this? We don't usually test depends_on blocks, so maybe the answer is no, I'm thinking about accidental regression.

[tannalynn]

Nice! I have a few questions, and the answers can all be no 🙂

• Should we add a changelog entry for this? It might be as close as we get to mentioning the new gem in the main agent's release notes.

I was thinking it would be better to wait until we've released a version of the traces-backend-newrelic before adding something for this to the changelog for whatever the next version of the agent is.

• Would it be helpful to have a comment around line 15 describing where Traces::Backend::NewRelic is defined? I'm worried someone might have trouble finding it since it's not part of the async-http library.

sure, added one

• Do we want to add a test for this? We don't usually test depends_on blocks, so maybe the answer is no, I'm thinking about accidental regression.

I was leaning towards not, since we dont normally and it'd add the time of running a whole new multiverse suite. Plus the consequence for it failing is pretty minimal. Since it just won't create the second spans inside of the traces-backend-newrelic gem, but the ones the agent creates would still be present, there isn't a concern about double reporting the external requests.

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SimpleCov Report

	Coverage	Threshold
Line	93.7%	93%
Branch	71.79%	71%