Provide NuGet package

[tomkerkhove]

Provide NuGet package so that we can mitigate the approach that was marked as obsolete in https://github.com/PrometheusClientNet/Prometheus.Client/releases/tag/4.0.0

[phnx47]

I prepared CI for deploy. We are waiting @sanych-sun for check my PR: #3

[phnx47]

First version: v0.1.0