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: policy_http_header.go
    path of file: ./repos/buildkit/vendor/github.com/Azure/azure-sdk-for-go/sdk/azcore/runtime
    
    the code of the file until where you have to start completion: //go:build go1.18
// +build go1.18

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

package runtime

import (
	"context"
	"net/http"

	"github.com/Azure/azure-sdk-for-go/sdk/azcore/internal/shared"
	"github.c
    
func httpHeaderPolicy(req *policy.Request) (*http.Response, error) {
	// check if any custom HTTP headers have been specified
	if header := req.Raw().Context().Value(shared.CtxWithHTTPHeaderKey{}); header != nil {
		for k, v := range header.(http.Header) {
			// use Set to replace any existing value
			// it also canonicalizes the header key
			req.Raw().Header.Set(k, v[0])
			// add any remaining values
			for i := 1; i < len(v); i++ {
				req.Raw().Header.Add(k, v[i])
			}
		}
	}
	return req.Next()
}