Support new metrics format for 10.0

[janhoy]

In Solr 10.0, released today, the Prometheus exporter is gone. Instead each solr node exposes prometheus-format metrics individually on /solr/admin/metrics.

Users on Solr 9.x should still be able to run PrometheusExporter as before which will work with Solr 9 but not solr 10.

Users moving to Solr 10, who want metrics, should upgrade to a new Solr Operator which supports configuring Prometheus to scrape solr endpoints intead of deploying solr-exporter.

The SolrClouds CRD and thus also the solr helm chart should make metrics exporting simple to use OOTB

• Default to open /solr/admin/metrics endpoints, document how to configure a BasicAuth user with access to this endpoint
• Add a new CRD key to enable prometheus ServiceMonitor pointing to the metrics endpoint, and make this key exposed as helm value
• Make the operator manage these ServiceMonitors

Users who want to push metrics to OTEL collector over OTLP can probably do this without custom operator support, by simply setting some environment variables to configure the OTEL SDK. Point users to relevant Solr documentation for this.