HttpHandlerDiagnosticListener: Write response event once when request is redirected

[lmolkova]

HttpHandlerDiagnosticListener was introduced in #18101. It allows to hook into the .NET Framework HttpClient implementation based on HttpWebRequest and get notifications with DiagnosticSource and inject correlation headers (similar to corefx instrumentation).

When HTTP request is redirected, the hook is invoked once per redirect. when request is being sent, the hook only sends one event for the first request and does not notify about each consecutive redirect.

When response is received, hook sends event for each response.

Consumers of these events uses them to log requests/responses and having the 'final' response code, actual duration and potentially details from the response is important. So every consumer for the hook needs to wait for the last response.

This fix implements it: it does not send Stop event

• if automatic redirects are allowed
• and response status code is one of the redirected ones
• and number of redirects is not reached maximum allowed

Otherwise it notifies consumer about the 'final' response.

/cc @vance @davidsh

[davidsh]

cc: @stephentoub