Add port number to the startup logs for the Prometheus Exporter

From a separate PR comment from janhoy:

Another nitpick not related to the move: Starting the exporter, it logs

INFO - 2022-01-24 21:34:21.636; org.apache.solr.prometheus.exporter.SolrExporter; Starting Solr Prometheus Exporting
Nowhere does it echo the port it was started on in the default case. Would be nice if the log said Started the exporter on port NNNN so folks trying it the first time will at least easily find it in browser.

https://github.com/apache/solr/pull/564#pullrequestreview-861551799