[RFE] Support Concurrent KVP Telemetry

[peytonr18]

Currently, the tracing-subscriber and general telemetry setup assumes all telemetry is emitted by azure-init. To better support external use, the tracing setup ought to allow libraries to initialize the subscriber with their metadata (ex: prefix with something other than azure-init).

This issue's goal is to bolster the current KVP setup to be more robust and support concurrent KVP emissions.

[cadejacobson]

It's also worth rexamining whether the default KVP should be emitting "Azure-init" (as it currently does) or "libazureinit" (where a third party would consume this feature from). If someone were to add the libazureinit package and not specify a new prefix, all KVP would read as coming from "Azure-init" which may not be ideal.