SOLR-15954: Move the Prometheus Exporter from modules

[HoustonPutman]

https://issues.apache.org/jira/browse/SOLR-15954

[janhoy]

I browsed the PR, built the branch, started Solr and then the exporter. Think this is ok.

I was going to builda Docker image from the binary distro to test the exporter from docker, but looking in docker/ folder in binary distro, there is no description on how to build the image. No description in main README.md either. I think there is one in the ref.guide, but should there not be a docker/README.md with a sample command?

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.

PS: The link to ref-guide in solr-exporter/README.md is broken link. Should be https://solr.apache.org/guide/8_11/monitoring-solr-with-prometheus-and-grafana.html, not https://solr.apache.org/guide/monitoring-with-prometheus-and-grafana.html as it is now.

[HoustonPutman]

I was going to builda Docker image from the binary distro to test the exporter from docker, but looking in docker/ folder in binary distro, there is no description on how to build the image. No description in main README.md either. I think there is one in the ref.guide, but should there not be a docker/README.md with a sample command?

I think this is a good point, but kind of separate from this ticket. I'll create a JIRA and do that separately.

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.

Again good point, but will fix separately.

PS: The link to ref-guide in solr-exporter/README.md is broken link. Should be https://solr.apache.org/guide/8_11/monitoring-solr-with-prometheus-and-grafana.html, not https://solr.apache.org/guide/monitoring-with-prometheus-and-grafana.html as it is now.

This is the name of the page starting with 9.0, so I agree that it isn't great that the link is broken until 9.0 is released. However, given that it will be released relatively soon, I think we can keep the future-proof URL.

[janhoy]

Great. Did not check the new refguide link, makes sense.