Add an attribute to specify unknown type before converting to gauge in prometheusreceiver

[khanhntd]

Description:
As Prometheus Receiver currently, all untyped prometheus metrics are converted to OTEL Gauge since the whole reason was not to drop untyped prometheus metrics. However, for some of the backend (such as CloudWatch) using with awsemf as an exporter/proxy, they will have limit bound (not supporting +inf or -inf) and combining with some case untyped can be counter, this won't work and will lose metrics. Therefore, there are two options that has been discussed in this issue:

• Dropping all untyped metrics with a flag in config
• Add a metric datapoint's attribute flag specify whether the metric's are unknown.

After consideration, the latter options would be more straightforward since we can use filterprocessor to filter/drop untyped metrics and also allow more metrics processing with untyped in exchange for more bytes (more than 16 bytes scaling per datapoint)

Another thoughts for improvement would be: if this attribute can also apply to others prometheus metrics type such as StateSet and Info since it has been converted to Otel Counter

Will update the spec after everyone agrees on this approach.

Link to tracking Issue:

• [receiver/prometheus] Add config option to allow dropping of untyped metrics #16768
• Support a cumulative "interpretation" of Unknown metrics wg-prometheus#69

Testing:
There are two tests I have done:

• go test ./... with prometheus receiver with update unit test

khanhntd@88665a0eaf54 prometheusreceiver % go test .
ok      github.com/open-telemetry/opentelemetry-collector-contrib/receiver/prometheusreceiver   219.046s

• Using awsemf along with prometheusreceiver with prometheus.go

package main

import (
        "log"
        "net/http"

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

var gaugeFunc = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
        Name: "gaugeFunc",
        Help: "Test for gauge functions",
},
        func() float64 {
                return float64(1.0)
        })

var noneFunc = prometheus.NewUntypedFunc(prometheus.UntypedOpts{
        Name: "noneFunc",
        Help: "Test for none functions",
},
        func() float64 {
                return float64(1.0)
        })
var summaryFunc = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
        Name: "summaryFunc",
        Help: "Test for summary functions",
},
        func() float64 {
                return float64(1.0)
        })

func main() {
        log.Printf("Registering all metrics...")
        prometheus.MustRegister(gaugeFunc)
        prometheus.MustRegister(noneFunc)
        prometheus.MustRegister(summaryFunc)
        log.Printf("Done registering all metrics...")

        http.Handle("/metrics", promhttp.Handler())
        http.ListenAndServe("0.0.0.0:8000", nil)

}

and have following results:

{
    "Version": "1",
    "_aws": {
        "CloudWatchMetrics": [
            {
                "Namespace": "CWAgent-Internal",
                "Dimensions": [
                    [
                        "key1",
                        "key2",
                        "prom_metric_type",
                        "job",
                        "instance",
                        "Version"
                    ]
                ],
                "Metrics": [
                    {
                        "Name": "noneFunc"
                    }
                ]
            }
        ],
        "Timestamp": 1671606728349
    },
    "instance": "127.0.0.1:8000",
    "job": "MY_JOB",
    "key1": "value1",
    "key2": "value2",
    "noneFunc": 1,
    "prom_metric_type": "unknown"
}

Documentation:

• To understand more about Prometheus Type, please follow this document
• Understand the conversion between OTEL and Prometheus

Signed-off-by: Khanh Nguyen khanhntd@amazon.com

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