You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: observer.go
    path of file: ./repos/fabio/vendor/github.com/openzipkin-contrib/zipkin-go-opentracing
    
    the code of the file until where you have to start completion: package zipkintracer

import (
	otobserver "github.com/opentracing-contrib/go-observer"
	opentracing "github.com/opentracing/opentracing-go"
)

// observer is a dispatcher to other observers
type observer struct {
	observers []otobserver.Observer
}

// spanObserver is a dispatcher to other span 
    
func (o observer) OnStartSpan(sp opentracing.Span, operationName string, options opentracing.StartSpanOptions) (otobserver.SpanObserver, bool) {
	var spanObservers []otobserver.SpanObserver
	for _, obs := range o.observers {
		spanObs, ok := obs.OnStartSpan(sp, operationName, options)
		if ok {
			if spanObservers == nil {
				spanObservers = make([]otobserver.SpanObserver, 0, len(o.observers))
			}
			spanObservers = append(spanObservers, spanObs)
		}
	}
	if len(spanObservers) == 0 {
		return nil, false
	}

	return spanObserver{observers: spanObservers}, true
}