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: metricobserver.go
    path of file: ./repos/juju/apiserver/observer/metricobserver
    
    the code of the file until where you have to start completion: // Copyright 2016 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package metricobserver

import (
	"net/http"
	"strconv"
	"time"

	"github.com/juju/clock"
	"github.com/juju/errors"
	"github.com/juju/names/v5"
	"github.com/prometheus/client_golang/prometheus"

	"github.com/juju/juju/apiserver/observer"
	"github.com/juju/juju/rpc"
)

// MetricLabels used for setting labels for the Counter and Summary vectors.
const (
	MetricLabelFacade    = "facade"
	Metr
    
func (o *rpcObserver) ServerReply(req rpc.Request, hdr *rpc.Header, body interface{}) {
	// The following reduces the number permutations around the cardinality.
	// All errors will be reported as "error" to remove this issue of exploding
	// out of quantiles.
	//
	// error_code of "" rather than "success" is to keep backwards compatible
	// with the existing grafana implementations.
	errorCode := ""
	if hdr.ErrorCode != "" {
		errorCode = "error"
	}

	labels := prometheus.Labels{
		MetricLabelFacade:    req.Type,
		MetricLabelVersion:   strconv.Itoa(req.Version),
		MetricLabelMethod:    req.Action,
		MetricLabelErrorCode: errorCode,
	}
	duration := o.clock.Now().Sub(o.requestStart)
	o.metrics.apiRequestDuration.With(labels).Observe(duration.Seconds())
}