NewSink constructor should accept an interface

https://pkg.go.dev/github.com/google/go-metrics-stackdriver#NewSink

currently only accepts a https://pkg.go.dev/cloud.google.com/go/monitoring/apiv3#MetricClient

which says:

```
Deprecated: Please use cloud.google.com/go/monitoring/apiv3/v2.
```

This was also mentioned https://github.com/google/go-metrics-stackdriver/pull/16 but would be a breaking change to the package.
