prometheus: unit is not appended to name

[gouthamve]

Description

The Prometheus compatibility spec specifies:

The resulting unit SHOULD be added to the metric as OpenMetrics UNIT metadata and as a suffix to the metric name unless the metric name already contains the unit, or the unit MUST be omitted. The unit suffix comes before any type-specific suffixes.

But this doesn't happen.

counter, err := meter.Float64Counter("foo", api.WithDescription("a simple counter"), api.WithUnit("s")) results in

# HELP foo_total a simple counter
# TYPE foo_total counter
foo_total{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version=""} 5

Environment

• OS: MacOS
• Architecture: darwin/arm64
• Go Version: 1.20.6
• opentelemetry-go version: 1.16.0

Steps To Reproduce

The exporter embeds a default OpenTelemetry Reader and
	// implements prometheus.Collector, allowing it to be used as
	// both a Reader and Collector.
	exporter, err := prometheus.New()
	if err != nil {
		log.Fatal(err)
	}
	provider := metric.NewMeterProvider(metric.WithReader(exporter))
	meter := provider.Meter("github.com/open-telemetry/opentelemetry-go/example/prometheus")

	// Start the prometheus HTTP server and pass the exporter Collector to it
	go serveMetrics()

	opt := api.WithAttributes(
		attribute.Key("A").String("B"),
		attribute.Key("C").String("D"),
	)

	// This is the equivalent of prometheus.NewCounterVec
	counter, err := meter.Float64Counter("foo", api.WithDescription("a simple counter"), api.WithUnit("s"))
	if err != nil {
		log.Fatal(err)
	}
	counter.Add(ctx, 5, opt)

	ctx, _ = signal.NotifyContext(ctx, os.Interrupt)
	<-ctx.Done()
}

func serveMetrics() {
	log.Printf("serving metrics at localhost:2223/metrics")
	http.Handle("/metrics", promhttp.Handler())
	err := http.ListenAndServe(":2223", nil)
	if err != nil {
		fmt.Printf("error serving http: %v", err)
		return
	}
}

[dashpole]

We do support units, but decided originally to only support the units defined by the Go SDK. I think we've since removed our constants for units, so it makes sense to expand what the prom exporter supports.