Prometheus exporter reports reader not registered on startup

[MrAlias]

Description

Running the Prometheus exporter example results in the following error being logged:

reader is not registered

Environment

• Go Version: 1.19
• opentelemetry-go version: b6a22ab

Steps To Reproduce

1. Checkout commit
2. cd example/prometheus
3. go run .
4. See error:

2022/10/14 13:23:14 reader is not registered
2022/10/14 13:23:14 serving metrics at localhost:2223/metrics

Expected behavior

The line 2022/10/14 13:23:14 reader is not registered should not be logged.

[MrAlias]

My guess is this error is returned by the underlying manualReader when the collector is registered. Likely because Describe is being called then and the exporter has not been registered with any MeterProvider.

Maybe this registration needs to be delayed?

[MadVikingGod]

After reading more details about the prometheus collector interface I think we should do nothing on Describe. This is called when the collector is registered with prometheus, and is intended to produce the names of the metrics that we will send prometheus. Because we won't know these until later, we should be using the unchecked collector mode of prometheus, by returning nothing.