Add authentication failure reason to AuthN metrics

[MackinnonBuck]

When authentication fails, the aspnetcore.authentication.request.duration histogram only includes the type of the exception that resulted in the failure. This doesn't provide very useful information about why authentication failed.

We should consider updating the error.type tag or adding a new tag (maybe aspnetcore.authentication.failure_reason) to specify a well-known reason why the authentication handler failed to authenticate. This tag would only apply to the aspnetcore.authentication.request.duration histogram.

See also: #59557 (comment)