Removes the functionality of the Describe in prometheus exporter. (#3342

)

* remove prom exporter Describe

* Apply suggestions from code review

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

CHANGELOG.md

@@ -16,6 +16,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - `sdktrace.TraceProvider.Shutdown` and `sdktrace.TraceProvider.ForceFlush` to not return error when no processor register. (#3268)
 - The `"go.opentelemetry.io/otel/exporters/prometheus".New` now also returns an error indicating the failure to register the exporter with Prometheus. (#3239)
+- The prometheus exporter will no longer try to enumerate the metrics it will send to prometheus on startup.
+   This fixes the `reader is not registered` warning currently emitted on startup. (#3291 #3342)
 
 ### Fixed
 

exporters/prometheus/exporter.go

@@ -69,13 +69,11 @@ func New(opts ...Option) (*Exporter, error) {
 
 // Describe implements prometheus.Collector.
 func (c *collector) Describe(ch chan<- *prometheus.Desc) {
-	metrics, err := c.reader.Collect(context.TODO())
-	if err != nil {
-		otel.Handle(err)
-	}
-	for _, metricData := range getMetricData(metrics) {
-		ch <- metricData.description
-	}
+	// The Opentelemetry SDK doesn't have information on which will exist when the collector
+	// is registered. By returning nothing we are an "unchecked" collector in Prometheus,
+	// and assume responsibility for consistency of the metrics produced.
+	//
+	// See https://pkg.go.dev/github.com/prometheus/client_golang@v1.13.0/prometheus#hdr-Custom_Collectors_and_constant_Metrics
 }
 
 // Collect implements prometheus.Collector.