Making Distributed Tracing configurable

[cmillar-salesloft]

Is your feature request related to a problem? Please describe. There appears to be no way to disable Distributed Tracing via the agent's configuration. Not all of our applications need Distributed Tracing so we'd like to disable it where it is not used, given the additional costs.

Describe the solution you'd like Other New Relic language agents allow for configuring whether Distributed Tracing is on or off. For example the Ruby agent allows clients to disable Distributed Tracing via an environment variable NEW_RELIC_DISTRIBUTED_TRACING_ENABLED, docs here. It would be great if the Elixir agent provided a way to configure this as well.

Describe alternatives you've considered N/A