Send metadata header on IMDS probe request. (Azure#15875)

* Call imds instance endpoint instead of token endpoint to probe for imds, extend timeout

* Update msi token tests to distinguish between which imds endpoint is being called

* Fix comment and visibility in test

* Change per stpetrov

* re-trigger checks

* Add metadata header to probe request

sdk/mgmtcommon/AppAuthentication/Azure.Services.AppAuthentication/TokenProviders/MsiAccessTokenProvider.cs

@@ -144,6 +144,7 @@ public override async Task<AppAuthenticationResult> GetAuthResultAsync(string re
                     {
                         string probeRequestUrl = $"{ImdsEndpoint}{ImdsInstanceRoute}?api-version={ImdsInstanceApiVersion}";
                         HttpRequestMessage imdsProbeRequest = new HttpRequestMessage(HttpMethod.Get, probeRequestUrl);
+                        imdsProbeRequest.Headers.Add(AzureVMImdsHeader, "true");
 
                         try
                         {