Remove scrape() from getPrometheusName() default implementation

[fstab]

The Collector (and MultiCollector) interface defines a method getPrometheusName() (or getPrometheusNames()) to get the list of metric names produced by the collector.

This method is optional. It is used during registration to check for duplicate metrics, and it is used during scrape if a name filter is present to check if this collector can be skipped because the name(s) are excluded.

The default implementation was to call collect() to learn the Prometheus names. However, this leads to two collect() calls when scraping: The first to check the name filter, the second for the actual scrape.

With this PR we change this to returning null (or empty list) indicating that the names are unknown.

This does not affect the metrics in prometheus-metrics-core, because all metrics in prometheus-metrics-core override this.