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: desc.go
    path of file: ./repos/faasd/vendor/github.com/prometheus/client_golang/prometheus
    
    the code of the file until where you have to start completion: // Copyright 2016 The Prometheus 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 License for the specific language governing permissions and
// limitations under the License.

package prometheus

import (
	"fmt"
	"sort"
	"strings"

	"github.com/cespare/xxhash/v2"
	dto "github.com/prometheus/client_model/go"
	"github.com/prometheus/common/model"
	"google.golang.org/protobuf/proto"

	"github.com/prometheus/client_golang/prometheus/internal"
)

// Desc is the descriptor used by every Prometheus Metric. It is essentially
// the immutable meta-data of a Metric. The normal Metric implementations
// included in this package manage their Desc under the hood. Users only have to
// deal with Desc if they use advanced features like the ExpvarCollector or
// custom Collectors and Metrics.
//
// Descriptors registered with the same registry have to fulfill certain
// consistency and uniqueness criteria if they share the same fully-qualified
// name: They must have the same help string and the same label names (aka label
// dimensions) in each, constLabels and variableLabels, but they must differ in
// the values of the constLabels.
//
// Descriptors that share the same fully-qualified names and the same label
// values of their constLabels are considered equal.
//
// Use NewDesc to create new Desc instances.
type Desc struct {
	// fqName has been built from Namespace, Subsystem, and Name.
	fqName string
	// help provides some helpful information about this metric.
	help string
	// constLabelPairs contains precalculated DTO label pairs based on
	// the constant labels.
	constLabelPairs []*dto.LabelPair
	// variableLabels contains names of labels
    
func (v2) NewDesc(fqName, help string, variableLabels ConstrainableLabels, constLabels Labels) *Desc {
	d := &Desc{
		fqName:         fqName,
		help:           help,
		variableLabels: variableLabels.compile(),
	}
	if !model.IsValidMetricName(model.LabelValue(fqName)) {
		d.err = fmt.Errorf("%q is not a valid metric name", fqName)
		return d
	}
	// labelValues contains the label values of const labels (in order of
	// their sorted label names) plus the fqName (at position 0).
	labelValues := make([]string, 1, len(constLabels)+1)
	labelValues[0] = fqName
	labelNames := make([]string, 0, len(constLabels)+len(d.variableLabels.names))
	labelNameSet := map[string]struct{}{}
	// First add only the const label names and sort them...
	for labelName := range constLabels {
		if !checkLabelName(labelName) {
			d.err = fmt.Errorf("%q is not a valid label name for metric %q", labelName, fqName)
			return d
		}
		labelNames = append(labelNames, labelName)
		labelNameSet[labelName] = struct{}{}
	}
	sort.Strings(labelNames)
	// ... so that we can now add const label values in the order of their names.
	for _, labelName := range labelNames {
		labelValues = append(labelValues, constLabels[labelName])
	}
	// Validate the const label values. They can't have a wrong cardinality, so
	// use in len(labelValues) as expectedNumberOfValues.
	if err := validateLabelValues(labelValues, len(labelValues)); err != nil {
		d.err = err
		return d
	}
	// Now add the variable label names, but prefix them with something that
	// cannot be in a regular label name. That prevents matching the label
	// dimension with a different mix between preset and variable labels.
	for _, label := range d.variableLabels.names {
		if !checkLabelName(label) {
			d.err = fmt.Errorf("%q is not a valid label name for metric %q", label, fqName)
			return d
		}
		labelNames = append(labelNames, "$"+label)
		labelNameSet[label] = struct{}{}
	}
	if len(labelNames) != len(labelNameSet) {
		d.err = fmt.Errorf("duplicate label names in constant and variable labels for metric %q", fqName)
		return d
	}

	xxh := xxhash.New()
	for _, val := range labelValues {
		xxh.WriteString(val)
		xxh.Write(separatorByteSlice)
	}
	d.id = xxh.Sum64()
	// Sort labelNames so that order doesn't matter for the hash.
	sort.Strings(labelNames)
	// Now hash together (in this order) the help string and the sorted
	// label names.
	xxh.Reset()
	xxh.WriteString(help)
	xxh.Write(separatorByteSlice)
	for _, labelName := range labelNames {
		xxh.WriteString(labelName)
		xxh.Write(separatorByteSlice)
	}
	d.dimHash = xxh.Sum64()

	d.constLabelPairs = make([]*dto.LabelPair, 0, len(constLabels))
	for n, v := range constLabels {
		d.constLabelPairs = append(d.constLabelPairs, &dto.LabelPair{
			Name:  proto.String(n),
			Value: proto.String(v),
		})
	}
	sort.Sort(internal.LabelPairSorter(d.constLabelPairs))
	return d
}