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

[Elanis]

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

[Elanis]

@dotnet-policy-service agree

[dotnet-policy-service]

Thanks for your PR, @@Elanis. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Elanis]

Feel free to ping me whenever someone in the team have time to review this :)
Also, I don't know what's the usual way to do things here since it's my first contribution, so feel free to guide me.

And finally, if this feature can be backported in .NET 9 I would be extremely happy as it's producing lots of erroneous data in my monitoring stack 😅 (thus why I noticed this)