Disable extended logging in DiagnosticsObserversInitializer by default

[dakanev-msft]

By default the DiagnosticsObserversInitializer logs extended details when the HttpClient receives an error response. In particular cases the log entry can contain the payload of the HTTP request message including Authorization and custom header values. This is a concern from privacy and security perspective as the header values might contain personally identifiable information and non encrypted access tokens.

Extended logging information should be off by default and should be explicitly enabled so that developers are in control of what is being logged.