Need a mode that will exercise all gem instrumentation without reporting data

[fallwith]

In a discussion in #2869, we discuss the need for running the agent in a non-production mode with all of the usual Ruby library instrumentation active but without streaming any data to New Relic that would incur cost or potentially confuse things on the UI side. I was under the impression that having agent_enabled: true and monitor_mode: false would give us everything we want, but evidently it doesn't.

We should consider supporting such a mode, either by making monitor_mode: false work that way or by introducing support for another usage pattern.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-321248