Update metric_types.md (#2238)
Update metric_types.md (#2238)

Updates the `metric_types.md` to include links to the .NET Client library.

Signed-off-by: Scott Pantall <a9eea0969be663b5f22ae455c25fb15e82f22634@gmail.com>

Signed-off-by: Scott Pantall <a9eea0969be663b5f22ae455c25fb15e82f22634@gmail.com>
prometheus/docs,prometheus/docs,prometheus/docs