AzureInstanceMetadata: replace tests

[TimothyMothra]

Attempting to switch to FrameworkReferences (#2355), and I found a problem with some tests for AzureInstanceMetadada.

What

unnecessary local server

First issue, the tests for AzureInstanceMetadata run a local server to intercept HttpRequests.
There were breaking changes to WebHostBuilder in the change from netcore2.x -> netcore3.x.

ApplicationInsights-dotnet/WEB/Src/WindowsServer/WindowsServer.Tests/Mock/AzureInstanceMetadataServiceMock.cs

Lines 27 to 36 in a90eb8f

	ResponseHandlerMock.OnRequestDictionary.Add(testName, onRequest);
	this.cts = new CancellationTokenSource();
	this.host = new WebHostBuilder()
	.UseKestrel()
	.UseStartup<ResponseHandlerMock>()
	.UseUrls(baseUrl + "?testName=" + testName)
	.Build();
	Task.Run(() => this.host.Run(this.cts.Token));

We do not need a local server to test AzureInstanceMetadata, we only need to mock the HTTP Response.
This is a good candidate to switch to use HttpClient mocking.
https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient.-ctor?view=net-5.0#System_Net_Http_HttpClient__ctor_System_Net_Http_HttpMessageHandler_

invalid tests

Second issue, tests were replacing the HTTP call with a Func<> that returns a parsed object.
The existing tests seem to verify the behavior of this mock class, and not the HttpClient.

While rewriting tests, I've found that the HttpClient successfully handles the test condition.

---

Affected product code:

AzureMetadataRequestor ctor accepts a Func<>.

ApplicationInsights-dotnet/WEB/Src/WindowsServer/WindowsServer/Implementation/AzureMetadataRequestor.cs

Lines 34 to 37 in a90eb8f

	internal AzureMetadataRequestor(Func<string, Task<AzureInstanceComputeMetadata>> makeAzureIMSRequestor = null)
	{
	this.azureIMSRequestor = makeAzureIMSRequestor;
	}

When making a request, it can switch to use the Func instead.
This is used in unit tests to invoke a local server.

ApplicationInsights-dotnet/WEB/Src/WindowsServer/WindowsServer/Implementation/AzureMetadataRequestor.cs

Lines 59 to 86 in a90eb8f

	private async Task<AzureInstanceComputeMetadata> MakeAzureMetadataRequestAsync(string metadataRequestUrl)
	{
	AzureInstanceComputeMetadata requestResult = null;
	SdkInternalOperationsMonitor.Enter();
	try
	{
	if (this.azureIMSRequestor != null)
	{
	requestResult = await this.azureIMSRequestor(metadataRequestUrl).ConfigureAwait(false);
	}
	else
	{
	requestResult = await this.MakeWebRequestAsync(metadataRequestUrl).ConfigureAwait(false);
	}
	}
	catch (Exception ex)
	{
	WindowsServerEventSource.Log.AzureInstanceMetadataRequestFailure(
	metadataRequestUrl, ex.Message, ex.InnerException != null ? ex.InnerException.Message : string.Empty);
	}
	finally
	{
	SdkInternalOperationsMonitor.Exit();
	}
	return requestResult;
	}

Affected Unit Tests:

Some of these tests are testing a condition that is only reproducible using the mock code.
I'm summarizing the affected tests here and will replace the tests where appropriate.

• AzureInstanceMetadataEndToEndTests
  • SpoofedResponseFromAzureIMSDoesntCrash
    tests that a request sucessfully parses the json response
  • AzureImsResponseTooLargeStopsCollection
    unable to force a failure here. HttpClient successfully handles the same json response object
  • AzureImsResponseExcludesMalformedValues
    this test invalid values within the json response object. these invalid values are correctly ignored.
  • AzureImsResponseTimesOut
    Unable to force a timeout using HttpClient. The HttpClient is sucessfully parsing the json response after timeout.
    Reading the docs, HttpClient will throw an exception when a timeout is experienced.
    While Testing, the AzureMetadataRequestor correctly handles exceptions.
  • AzureImsResponseUnsuccessful
    test crrectly verifies errant HttpStatusCodes.
• AzureInstanceMetadataTests
  • AzureIMSGetFailsWithException
    this test verifies that the AzureMetadataRequestor handles HttpExceptions.