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: main.go
    path of file: ./repos/opencensus-go/examples/http/helloworld_server
    
    the code of the file until where you have to start completion: // Copyright 2018, OpenCensus Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the Li
    
func main() {
	// Start z-Pages server.
	go func() {
		mux := http.NewServeMux()
		zpages.Handle(mux, "/debug")
		log.Fatal(http.ListenAndServe("127.0.0.1:8081", mux))
	}()

	// Using log exporter to export metrics but you can choose any supported exporter.
	exporter, err := exporter.NewLogExporter(exporter.Options{
		ReportingInterval: 10 * time.Second,
		MetricsLogFile:    metricsLogFile,
		TracesLogFile:     tracesLogFile,
	})
	if err != nil {
		log.Fatalf("Error creating log exporter: %v", err)
	}
	exporter.Start()
	defer exporter.Stop()
	defer exporter.Close()

	// Always trace for this demo. In a production application, you should
	// configure this to a trace.ProbabilitySampler set at the desired
	// probability.
	trace.ApplyConfig(trace.Config{DefaultSampler: trace.AlwaysSample()})

	// Report stats at every second.
	view.SetReportingPeriod(1 * time.Second)

	client := &http.Client{Transport: &ochttp.Transport{}}

	http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
		fmt.Fprintf(w, "hello world")

		// Provide an example of how spans can be annotated with metadata
		_, span := trace.StartSpan(req.Context(), "child")
		defer span.End()
		span.Annotate([]trace.Attribute{trace.StringAttribute("key", "value")}, "something happened")
		span.AddAttributes(trace.StringAttribute("hello", "world"))
		time.Sleep(time.Millisecond * 125)

		r, _ := http.NewRequest("GET", "https://example.com", nil)

		// Propagate the trace header info in the outgoing requests.
		r = r.WithContext(req.Context())
		resp, err := client.Do(r)
		if err != nil {
			log.Println(err)
		} else {
			// TODO: handle response
			resp.Body.Close()
		}
	})
	log.Fatal(http.ListenAndServe(":50030", &ochttp.Handler{}))
}