[Service Bus] Respect env variables listed in the guidelines

[ramya-rao-a]

The section on environment variables in the guidelines require us to load the relevant settings from the below environment variables, but Service Bus doesnt do so today.

@bterlson Would you suggest that this be done by the library independent of the efforts for the common configuration service?

[ramya-rao-a]

Updated link: https://github.com/Azure/azure-sdk/blob/master/_includes/tables/environment_variables.md

The ones under Identity section is handled by the @azure/identity package.
AZURE_LOG_LEVEL is already taken care of in the logger package.

@xirzec, Can AZURE_TRACING_DISABLED be handled in a central location i.e. in the core-tracing package?

#5696 tracks the rest for SDKS for http based services

The ones applicable to Event Hubs & Service Bus are the proxy related env vars only.
But, one needs to choose a WebSocket implementation, which takes the proxy related info when wanting to work with proxy. So, the Event Hubs & the Service Bus packages cannot implicitly make use of the environment variables to work with proxy.

[xirzec]

I'm actually curious what we should do in the case that AZURE_TRACING_DISABLED is set but the user passes us a valid tracer and parent span.

Certainly we could make a helper for looking at the variable and put that in core-tracing, though I'm less certain about the best way to implement since today we use a NoopSpans in the null case. Maybe if that variable is set, we simply throw if you try to set a global tracer?

[ramya-rao-a]

Maybe if that variable is set, we simply throw if you try to set a global tracer?

I should be able to tweak the env variable and disable tracing without having to change my code.
So, my vote is to not throw and use the no-op tracer

[ramya-rao-a]

Logged #10205 to track the work on AZURE_TRACING_DISABLED

[ramya-rao-a]

Closing as no further work is pending here