[FeatureRequest] Add a flag to turn off the default customer logging

[Hazhzeng]

With the aid of Python Worker Extension and Open Census Extension, customers are expecting full control on logging with third-party libraries. However, since Function Host attaches default logger in Python Worker, this results in duplication on messages in Application Insight.

We want to introduce a feature flag to suspend customer logs and let the third-party libraries to do so. Warning: we still want system logs to go through

This is an ask from https://github.com/Azure-Samples/azure-monitor-opencensus-python/pull/7/files.