HostingMetrics: empty "http.route" tags shouldn't be present

When pushing metrics to third-party softwares such as prometheus. Having empty-valued labels will cause issues

Description

Microsoft.AspNetCore.Hosting create a metric called http.server.request.duration documented here: https://learn.microsoft.com/en-us/aspnet/core/log-mon/metrics/built-in

One the available labels is http.route which is optional.

Issue:

http.route can be populated with various string such as:

• path/to/my/route/{id}
• index
• api/controller/route
• <empty string>
• ...

And it can also be missing.

However, based on the OpenMetrics spec used by Prometheus, empty label values should be treated as if the label was not present.
That means when exporting data to prometheus, we currently create two different metrics (one for http_route= <empty string>, one for the missing http_route), which create a "duplicate sample for timestamp" error when importing data.

Fix:

To fix this issue, I changed the metric labelling so an empty route doesn't create the label, as if the variable was null.

Fixes #62431