Pre-Register Server & Client Metrics

Following the guidance of Prometheus' best practices (https://prometheus.io/docs/practices/instrumentation/#avoid-missing-metrics), this library should pre-register its client/server interceptor metrics.

The Go version currently does this successfully (see `InitializeMetrics` here: https://github.com/grpc-ecosystem/go-grpc-prometheus/blob/master/server_metrics.go#L132)

The problem faced by not pre-registering is, for example, illustrated below:

```
grpc_server_handled_total{code="OK"}
```

In this example, even if there were zero requests handled with `code=OK`, the value should be `0` as long as these server interceptor metrics were being scraped. Another example:

```
sum by(instance)(rate(grpc_server_handled_total{code!~"OK"}[5m]))
/
sum by(instance)(rate(grpc_server_handled_total[5m]))
```

Here we're dividing "bad requests" by "total requests". If the service were 100% healthy, this would return `NaN`. It's expected that this would only result in `NaN` iff there have been exactly 0 requests total (as we'd still be dividing by 0 even after pre-registration). In any other case, when there's been at least one request, this would not equal `NaN` (regardless of the values for `code`).

While there are workarounds, pre-registration seems like the ideal fix here.

Additional resources:
- https://github.com/grpc-ecosystem/go-grpc-prometheus/issues/2
- https://www.robustperception.io/existential-issues-with-metrics