Add system.diagnostics.metrics.meter support

[droosma]

After our project started using OpenTelemetry I started using this package. In an effort to get some insights into how the system was running we introduced some Custom Metrics. We found them usefull, so I thought I would contribute them back.

Please take a look to see if you would be interested in these changes, I'm open to your suggestions of improvement.

Thank you for your time.

[mookid8000]

Excellent, thanks 👍 it's out in Rebus.OpenTelemetry 1.0.0-alpha02 on NuGet.org now 🙂