testutil.CollectAndCompare fails for metrics with empty help string

[bluekeyes]

Version: 1.16.0

While trying to write a test for a custom collector, I discovered that the testutil.CollectAndCompare function does not work with metrics that have an empty help string. The issue appears to be a discrepancy between the encoding and decoding of the text format:

1. When encoding metrics from a registry (i.e. the "got" side), the registry always sets Help to a non-nil value.
2. If the Help field is non-nil, the text encoding writes a HELP line
3. When parsing metrics from text (i.e. the "want" side), the text encoding does not populate the Help field if the line only contains whitespace after the keyword.

This example program shows how the comparison fails regardless of whether the HELP comment appears in the expected text:

Example

package main

import (
	"fmt"
	"strings"

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

type Collector struct{}

func (c Collector) Describe(ch chan<- *prometheus.Desc) {}

func (c Collector) Collect(ch chan<- prometheus.Metric) {
	ch <- prometheus.MustNewConstMetric(
		prometheus.NewDesc("sample_metric", "", nil, nil),
		prometheus.GaugeValue,
		0,
	)
}

func main() {
	withHelp := `
# HELP sample_metric
# TYPE sample_metric gauge
sample_metric 0
`

	withoutHelp := `
# TYPE sample_metric gauge
sample_metric 0
`
	fmt.Println("--- With HELP ---")
	fmt.Print(testutil.CollectAndCompare(Collector{}, strings.NewReader(withHelp)))

	fmt.Println()
	fmt.Println("--- Without HELP ---")
	fmt.Print(testutil.CollectAndCompare(Collector{}, strings.NewReader(withoutHelp)))
}

My guess is that the test functions should normalize empty help fields before comparing metrics, rather than changing how encoding or decoding works. If that sounds reasonable, I can look at submitting a PR.

[RubanChristian]

JS

[bwplotka]

👍🏽 Thanks for this, indeed let's improve it