Adding label to golang prometheus collector

Question

I'm trying to figure out how to add a label to a prometheus collector. Any ideas what I'm missing here? I have two files: main.go and collector.go

I used the following link as a guide. https://rsmitty.github.io/Prometheus-Exporters/

I mocked up this example, so I could post it here. I'm ultimately not going to pull "date +%s" for the command. Just can't figure out where to add labels.

For the label I'm trying to add a hostname, so I have a result like:

# HELP cmd_result Shows the cmd result
# TYPE cmd_result gauge
cmd_result{host="my_device_hostname"} 1.919256141363144e-76

I'm also really new to golang, so there is a good chance I'm going about this all wrong. I'm ultimately trying to get prometheus to pull the cmd result on each scrape.

main.go

package main

import (
    "net/http"

    log "github.com/Sirupsen/logrus"
    "github.com/prometheus/client_golang/prometheus"
    "github.com/prometheus/client_golang/prometheus/promhttp"
)

func main() {

    //Create a new instance of the collector and
    //register it with the prometheus client.
    cmd := newCollector()
    prometheus.MustRegister(cmd)

    //This section will start the HTTP server and expose
    //any metrics on the /metrics endpoint.
    http.Handle("/metrics", promhttp.Handler())
    log.Info("Beginning to serve on port :8080")
    log.Fatal(http.ListenAndServe(":8080", nil))
}

collector.go

package main

import (
    "encoding/binary"
    "fmt"
    "math"
    "os/exec"
    "strings"

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

type cmdCollector struct {
    cmdMetric *prometheus.Desc
}

func newCollector() *cmdCollector {
    return &cmdCollector{
        cmdMetric: prometheus.NewDesc("cmd_result",
            "Shows the cmd result",
            nil, nil,
        ),
    }
}

func (collector *cmdCollector) Describe(ch chan<- *prometheus.Desc) {
    ch <- collector.cmdMetric
}

func (collector *cmdCollector) Collect(ch chan<- prometheus.Metric) {

    var metricValue float64
    command := string("date +%s")
    cmdResult := exeCmd(command)
    metricValue = cmdResult

    ch <- prometheus.MustNewConstMetric(collector.cmdMetric, prometheus.GaugeValue, metricValue)

}

func exeCmd(cmd string) float64 {
    parts := strings.Fields(cmd)
    out, err := exec.Command(parts[0], parts[1]).Output()
    if err != nil {
        fmt.Println("error occured")
        fmt.Printf("%s", err)
    }
    cmdProcessResult := Float64frombytes(out)
    return cmdProcessResult
}

func Float64frombytes(bytes []byte) float64 {
    bits := binary.LittleEndian.Uint64(bytes)
    float := math.Float64frombits(bits)
    return float
}

Answer 1

I figured it out. I had to declare the label where I was calling the NewDesc method and then pass the value within the MustNewConstMetric method

Here is my new "newCollector" with the "hostname" label.

func newCollector() *cmdCollector {
    return &cmdCollector{
        cmdMetric: prometheus.NewDesc("cmd_result",
            "Shows the cmd result",
            []string{"hostname"}, nil,
        ),
    }
}

It's worth noting that I'm only adding "variable labels" here. That last 'nil' is for constant labels.

You can add any number of items like so...

[]string{"hostname", "another_label", "and_another_label"}

This is covered here: https://godoc.org/github.com/prometheus/client_golang/prometheus#NewDesc

Next I can add those values when calling the "MustNewConstMetric" method.

ch <- prometheus.MustNewConstMetric(collector.cmdMetric, prometheus.GaugeValue, metricValue, hostname)

The whole block...

func (collector *cmdCollector) Collect(ch chan<- prometheus.Metric) {

    var metricValue float64
    command := string("date +%s")
    cmdResult := exeCmd(command)
    metricValue = cmdResult

    ch <- prometheus.MustNewConstMetric(collector.cmdMetric, prometheus.GaugeValue, metricValue, hostname)

}

If I was passing in multiple labels; such as my example above, it would look more like this...

ch <- prometheus.MustNewConstMetric(collector.cmdMetric, prometheus.GaugeValue, metricValue, hostname, anotherLabel", "andAnotherLabel)

This is covered here: https://godoc.org/github.com/prometheus/client_golang/prometheus#MustNewConstMetric

Answer 2

The github.com/prometheus/client_golang library is non-trivial to use. I'd suggest taking a look at much simpler Go library for exporting metrics in Prometheus format - github.com/VictoriaMetrics/metrics . You can easily specify arbitrary number of dynamic labels via fmt.Sprintf() function from standard Go library:

import (
  "github.com/VictoriaMetrics/metrics"
)

func exportCMDResultValue(hostname string, metricValue float64) {
  metricName := fmt.Sprintf("cmd_result{host=%q}", hostname)
  metrics.GetOrCreateFloatCounter(metricName).Set(metricValue)
}

Then the corresponding cmd_result{host="<hostname>"} <metricValue> metric will be exported at /metrics page each time exportCMDResultValue() function is called. The /metrics page handler can be implemented with the following code:

http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
    metrics.WritePrometheus(w, false)
})

See WritePromtheus function docs for details.