Upgrade prometheus-net to v3.x

[Daniel15]

What this PR does / why we need it:
Upgrades the Prometheus integration to use v3 of the prometheus-net library.

This is the smallest amount of code needed to change in order to upgrade prometheus-net and keep everything in its current state.

To test it, I enabled the Prometheus metric endpoint in the HealthChecks.Sample project.

There's a bit of weirdness in this project's integration into Prometheus - Instead of having its own separate endpoint that updates the metrics and returns them in Prometheus format, it should instead rely on the standard /metrics Prometheus endpoint exposed by endpoints.MapMetrics() from prometheus-net.AspNetCore, and integrate into Prometheus by using Registry.AddBeforeCollectCallback (so that Prometheus calls this library to update the metrics, rather than going the other way around). However, that can be done in a separate PR.

Please reference the issue this PR will close:
Fixes #419

Special notes for your reviewer:
N/A

Does this PR introduce a user-facing change?:
This is a breaking change as v3 of prometheus-net is not API-compatible with v2. People just using AspNetCore.Diagnostics.HealthChecks without any customisations to Prometheus (eg. adding their own metrics) will be fine, but people that have deeper integrations into Prometheus will need to upgrade their own code to use the v3 API.

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[unaizorrilla]

Looks good, I try to review/merge this week!

[unaizorrilla]

Hi @Daniel15

Thanks for this contribution, a new release build is on appveyor, when finished a new packages 3.1.3 will be on NuGet with these changes!