Allow logging level to be set through the environment

[mattbaron]

I will be submitting a PR for this.

[mattbaron]

@Esity I'm waffling a little bit on how to do this, partly because the default behavior in regard to the logging level varies a little bit.

If Telemetry::Logger::setup is called, the default level is info, although it can be customized through the setup call. If Telemetry::Logger::setup isn't called, the default logging level is debug. Is this by design? Can that be changed to be a uniform default regardless of scenario? Or would you rather keep the functionality as-is? I want to add this ENV variable functionality while being careful of not changing the behavior too much.