Adds a Producer Interceptor example

[d1egoaz]

add some examples for the usage of the new interceptors feature added in #1730

[varun06]

one nit about documentation. Look great.
Do we need to push binary as part of these examples?

[d1egoaz]

@varun06 I don't think we need to push binaries none of the examples are doing that 🤷
binaries can also be easily crated as a go.mod is provided

[danielhookx]

The situation I met is that I want to put the parent span in the context, then the Kafka Producer interceptor to create the child span from this context. But It seems not supported by getting the context from the OnSend method.

The code looks like:

func Caller() {
        ...
        span := tracer.StartSpan(info.FullMethod)
	defer span.Finish()
	ctx = opentracing.ContextWithSpan(ctx, span)
        ....
        Publish(ctx)
        ....
}

func Publish(ctx context.Context) {
      var kafkaPub sarama.SyncProducer
       ...
       m := &sarama.ProducerMessage{
		Key:   sarama.StringEncoder(fromId),
		Topic: appTopic,
		Value: sarama.ByteEncoder(b),
	}
	_, _, _ = kafkaPub.SendMessage(ctx, m)
}

type ProducerInterceptor struct {
}

func (oi *ProducerInterceptor) OnSend(ctx context.Context, msg *sarama.ProducerMessage) {
	span, ctx := opentracing.StartSpanFromContext(ctx, method)
	defer span.Finish()
        ...
}

So please add some more information about it or is there any place I could find the right example to solve it?