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: json_formatter.go
    path of file: ./repos/cointop/vendor/github.com/sirupsen/logrus
    
    the code of the file until where you have to start completion: package logrus

import (
	"bytes"
	"encoding/json"
	"fmt"
	"runtime"
)

type fieldKey string

// FieldMap allows customization of the key names for default fields.
type FieldMap map[fieldKey]string

func (f FieldMap) resolve(key fieldKey) string {
	if k, ok := f[key]; ok {
		return k
	}

	return string(key)
}

// JSONFormatter formats logs into parsable json
type JSONFormatter struct {
	// TimestampFormat sets the format used for marshaling timestamps.
	// The format to use is the same than for time.Format or time.Parse from the standard
	// library.
	// The standard Library already provides a set of predefined format.
	TimestampFormat string

	// DisableTimestamp allows disabling automatic timestamps in output
	DisableTimestamp bool

	// DisableHTMLEscape allows disabling html escaping in output
	DisableHTMLEscape bool

	// DataKey allows users to put all the log entry parameters into a nested dictionary at a given key.
	DataKey string

	// FieldMap allows users to customize the names of keys for default fields.
	// As an example:
	// formatter := &JSONFormatter{
	//   	FieldMap: FieldMap{
	// 		 FieldKeyTime:  "@timestamp",
	// 		 FieldKeyLevel: "@level",
	// 		 FieldKeyMsg:   "@message",
	// 		 FieldKeyFunc:  "@caller",
	//    },
	// }
	FieldMap FieldMap

	// CallerPrettyfier can be set by the user to modify the content
	// of the function and file keys in the json data when ReportCaller is
	// activated. If any of the returned value is the empty string the
	// corresponding key will be removed from json fields.
	CallerPrettyfier func(*runtime.Frame) (function string, file string)

	// PrettyPrint will indent all json
    
func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
	data := make(Fields, len(entry.Data)+4)
	for k, v := range entry.Data {
		switch v := v.(type) {
		case error:
			// Otherwise errors are ignored by `encoding/json`
			// https://github.com/sirupsen/logrus/issues/137
			data[k] = v.Error()
		default:
			data[k] = v
		}
	}

	if f.DataKey != "" {
		newData := make(Fields, 4)
		newData[f.DataKey] = data
		data = newData
	}

	prefixFieldClashes(data, f.FieldMap, entry.HasCaller())

	timestampFormat := f.TimestampFormat
	if timestampFormat == "" {
		timestampFormat = defaultTimestampFormat
	}

	if entry.err != "" {
		data[f.FieldMap.resolve(FieldKeyLogrusError)] = entry.err
	}
	if !f.DisableTimestamp {
		data[f.FieldMap.resolve(FieldKeyTime)] = entry.Time.Format(timestampFormat)
	}
	data[f.FieldMap.resolve(FieldKeyMsg)] = entry.Message
	data[f.FieldMap.resolve(FieldKeyLevel)] = entry.Level.String()
	if entry.HasCaller() {
		funcVal := entry.Caller.Function
		fileVal := fmt.Sprintf("%s:%d", entry.Caller.File, entry.Caller.Line)
		if f.CallerPrettyfier != nil {
			funcVal, fileVal = f.CallerPrettyfier(entry.Caller)
		}
		if funcVal != "" {
			data[f.FieldMap.resolve(FieldKeyFunc)] = funcVal
		}
		if fileVal != "" {
			data[f.FieldMap.resolve(FieldKeyFile)] = fileVal
		}
	}

	var b *bytes.Buffer
	if entry.Buffer != nil {
		b = entry.Buffer
	} else {
		b = &bytes.Buffer{}
	}

	encoder := json.NewEncoder(b)
	encoder.SetEscapeHTML(!f.DisableHTMLEscape)
	if f.PrettyPrint {
		encoder.SetIndent("", "  ")
	}
	if err := encoder.Encode(data); err != nil {
		return nil, fmt.Errorf("failed to marshal fields to JSON, %w", err)
	}

	return b.Bytes(), nil
}