Relative Paths problem in Azure East/West deployment

[mattisking]

Please, fill the following sections to help us fix the issue

What happened: TLDR: The code that works out full paths from relative paths for the healthcheck in the healthcheck-ui isn't quite working when deployed as an App Service in Azure. This results in a "System.Net.Sockets.SocketException" "No such host is known." every time the UI calls the healthcheck.
"HealthChecksUI": {
"HealthChecks": [
{
"Name": "HealthChecksService",
"Uri": "/healthcheck"
}
],
"Webhooks": [],
"EvaluationTimeinSeconds": 10
},

GetHealthReport threw an exception when trying to get report from /healthcheck configured with name HealthChecksService.

This results in the UI failing to pull any health report.

What you expected to happen: I'd expect a health report.

How to reproduce it (as minimally and precisely as possible):

1. Use a config like above.
2. Deploy to Azure as an App Service.

Source code sample: I can create a sample if needed, but even the samples will do the same.

Anything else we need to know?: I am specifically using 3.1.3 of the libraries. Since it's Azure and an App Service it's not using Kestrel, it's using IIS.

I pulled down the code for 3.1.3, removed the nuget references and added UI, UI.Client, and UI.Core to my project and added it as a project reference, cleaning that up to build in our project and aligning .Net Core libraries and references. I also modified:
HealthChecks.UI
Core/HostedService/HealthCheckReportCollector.cs
private async Task GetHealthReport(HealthCheckConfiguration configuration)
in the exception to be a little more useful:
108: _logger.LogError(exception, $"GetHealthReport threw an exception when trying to get report from {uri} configured with name {name}: absoluteUri: {absoluteUri}.");
... by adding the Uri it's trying to use after working out the relative paths.

What's happening is this:
We have an east and west and the Azure generated URL for each is this:
https://oursite-dev-east-site.azurewebsites.net/
https://oursite-dev-west-site.azurewebsites.net/

So, I'd expect when I use /healthcheck I'd get:
https://oursite-dev-east-site.azurewebsites.net/healthcheck
https://oursite-dev-west-site.azurewebsites.net/healthcheck

But in fact, the relative path is being worked out as:
https://oursite-dev-east-site/healthcheck
https://oursite-dev-west-site/healthcheck

It's missing ".azurewebsites.net" so the httpclient is failing with "No Such Host."

**I'd recommend adding absoluteUri to the error logging as that's clearly something that's needed here when things fail.

Environment: Azure East/West typical App Service deployment.

• .NET Core version 3.1
• Healthchecks version 3.1.3
• Operative system: Windows
• Others:

[Ericvf]

It might be related to this as well: #1779