AddMetricFactory should use Metrics.DefaultCollectorRegistry unless specified otherwise

[tomkerkhove]

AddMetricFactory should use Metrics.DefaultCollectorRegistry unless specified otherwise because otherwise it breaks the integration with UsePrometheusServer

[phnx47]

This is early version by @sanych-sun. I will make my correction.

[sanych-sun]

I think it will be better to do it in opposite direction. UsePrometheusServer should be updated to the following algorithm:

1. use the registry provided in options
2. try to resolve the registry from container
3. fallback to legacy static entry point

I'll update Prometheus.Client.AspNetCore and Prometheus.Client.MetricServer with this logic soon as well as examples.

[tomkerkhove]

Fine by me, but need something as it was not working 😅