Request Body disposed in EndRequest diagnostic event

### Describe the bug
The Request Body stream is disposed at some point before the `Microsoft.AspNetCore.Hosting.EndRequest` diagnostic event is fired. That means any diagnostic listener on the end request - including Application Insight's telemetry for requests - cannot read the body.

### To Reproduce
Steps to reproduce the behavior:
1. Using this version of ASP.NET Core '2.2.4'
2. Run this code

RequestDiagnosticListener:
```csharp
public class RequestDiagnosticListener
{
[DiagnosticName("Microsoft.AspNetCore.Hosting.BeginRequest")]
public virtual void OnRequestBeginning(HttpContext httpContext)
{
httpContext.Request.EnableBuffering();
if (httpContext.Request.Body.CanSeek)
{
var position = httpContext.Request.Body.Position;
}
}

[DiagnosticName("Microsoft.AspNetCore.Hosting.EndRequest")]
public virtual void OnRequestEnding(HttpContext httpContext)
{
if (httpContext.Request.Body.CanSeek)
{
var position = httpContext.Request.Body.Position;
}
}
}
```
Startup:
```csharp
public void Configure(DiagnosticListener diagnosticListener)
{
diagnosticListener.SubscribeWithAdapter(new RequestDiagnosticListener());
}
```
3. With any HTTP request to the application
4. Getting the position in `OnRequestBeginning` works as desired. See error `System.ObjectDisposedException: 'Cannot access a closed Stream.'` on `OnRequestEnding` access to the `Position` property.

### Expected behavior
Request Body should be able to be read in the `Microsoft.AspNetCore.Hosting.EndRequest` diagnostic event. If the Body is to be disposed, that should happen after the EndRequest event.

### Additional context
This is a continuation of an issue reported on Application Insights: https://github.com/Microsoft/ApplicationInsights-aspnetcore/issues/686

In .NET Framework, an Application Insights Telemetry Initializer for RequestTelemetry is able to read the content of the request. That functionality has been lost in AspNetCore.