feat: Add env var to control sending telemetry to appinsights in functions worker

[lzchen]

Follow up to #1509

We have decided to separate out the context propagation and enabling azure monitor functionality. To do this, we will be using a separate env var PYTHON_APPLICATIONINSIGHTS_ENABLE_TELEMETRY, which will be for strictly sending telemetry to appinsights and as well enabling context propagation. Future configuration of the telemetry SDK will most likely have the word _APPLICATIONINSIGHTS_ inside it's name.

We treating Opentelemetry context propagation as a separate feature. This will be controlled as an opt-in like before with PYTHON_ENABLE_OPENTELEMETRY env var.

[MatteoMilanese]

If both appinsights_app_setting and opentelemetry_app_setting are set to "true", configure_azure_monitor could be called twice, first by user then by Dispatcher.
Could this be a potential bug? In case it could be, shouldn't the settings be exclusive with opentelemetry_app_setting having higher priority?

[lzchen]

@MatteoMilanese

configure_azure_monitor could be called twice, first by user then by Dispatcher.

Good question. We do not safeguard against double instrumentation as this is an unsupported scenario. We advise customers to either turn the appsetting on OR use configure_azure_monitor() in their code.