Fix the sample code in Metrics for Pulsar Functions document

[yumochiz]

Motivation

In https://pulsar.incubator.apache.org/docs/latest/functions/metrics/
introducing the sample code of using metric.

import org.apache.pulsar.functions.api.Context;
import org.apache.pulsar.functions.api.Function;

public class MetricRecordingFunction implements Function<String, Void> {
    @Override
    public void apply(String input, Context context) {
        context.recordMetric("number-of-characters", input.length());
        return null;
    }
}

I cannot compile the code because override method is different.

Modifications

I changed method apply method to process method.
I also change void to Void.

@Override
public Void process(String input, Context context) {
    context.recordMetric("number-of-characters", input.length());
    return null;
}

Result

We can compile the code.

[nkurihar]

run cpp tests

[nkurihar]

run integration tests

[nkurihar]

run cpp tests