ContextManager.ActiveSpan is null in ApplicationOnEndRequest

[DevinShaw]

Please answer these questions before submitting your issue.

• Why do you submit this issue?
• Question or discussion
• Bug
• Requirement
• Feature or performance improvement

Bug

• Which version of SkyWalking, OS and .NET Runtime?
  SkyWalking: 0.7.0
  OS: win7
  .NET Runtime: 4.6.1

• Which project?
  SkyWalking.Sample.AspNet

• What happen?
  When I add async action to ValuesController in SkyWalking.Sample.AspNet, I find that ContextManager.ActiveSpan is always null.

    public void ApplicationOnEndRequest(object sender, EventArgs e)
    {
        var httpApplication = sender as HttpApplication;
        var httpContext = httpApplication.Context;
        if (httpContext.Request.HttpMethod == "OPTIONS")
        {
            //asp.net Exclude OPTIONS request
            return;
        }
        var httpRequestSpan = ContextManager.ActiveSpan ;
        if (httpRequestSpan == null)
        {
            return;
        }
    }

[liuhaoyang]

This is due to the AsyncLocal cross-threading bug in the .NET Framework.

[liuhaoyang]

Repeat with #102, this bug will be fixed in V0.8.