there is no API to read collection properties

Solr 7.3 added the COLLECTIONPROP API call (https://lucene.apache.org/solr/guide/7_3/collections-api.html#collectionprop) that allows to set arbitrary properties on a collection. There is however no API call that returns the data. The only option is to manually read out the collectionprops.json file in ZK below the collection.

Options could be that the COLLECTIONPROP command has an option to retrieve properties, have a special command to list the properties and/or to have the properties listed in the clusterstatus output for a collection.

Would be great if SolrJ would also be supported.