Add support to parse self-diagnostics settings from environment variables on Windows.

[Yun-Ting]

Application Insights SDK offers new self-diagnostics that allows customers to turn on and turn off SDK tracing without requiring an application restart.

The problem we have is that the SDK only checks the working directory for the existence of this JSON file - ApplicationInsights-dotnet/SelfDiagnosticsConfigParser.cs at 405fd6a9916956f2233520c8ab66110a1f9dcfbc · microsoft/ApplicationInsights-dotnet (github.com) This is good enough for most host environments however for Function Apps environment that working directory is a folder path where customers are not allowed to drop in a config file.

If any of our SDKs offer self-diagnostic tracing that allows us to turn on and turn off tracing which gets EventSource type information about how the SDK is performing, producing telemetry, and able or not able to send that telemetry to ingestion then we'd like to request an ability to set what directory the SDK will search for this configuration file by checking an environment variable.

If the location where SDK checks for this configuration file is configurable from environment variable, then we can ask Function App Customers to set that Env Var (using the configuration tab in Azure portal on their function) to a path they do have write access to and allows our SDK to pickup and detect the self-diagnostic settings so that we can get some type of client visibility into how our SDK is performing in function app dedicated runtime hosts.