Log a warning message when scope is not available

[gaelreyrol]

Some components can only be traced in specific condition where a scope has already been activated, such as the mailer or the messenger components which can only be used from specific entry points like a request, a command or a worker.

We warn the user if there is no scope to rely on which would cause spans to look for a root span which would not exits.

[gaelreyrol]

This warning should be configurable with the option open_telemetry.instrumentation.no_root_span_warning for example.

[gaelreyrol]

Currently a debug message is logged when no active scope/root span is available. I think this is enough to investigate if spans are being created without root spans. I might reconsider this issue later when I will have enough feedback to implement it.