The underlying issue is SOLR-7521, a Lucene 5 bug / "feature" that prevents a string index view of an indexed numeric field.
This issue will address SOLR-7496 for single valued numeric fields.
Here's a patch that adds FacetFieldProcessorNumeric that uses the new slot accumulator resize ability in conjunction with a hash table.
Commit 1678535 from Yonik Seeley in branch 'dev/trunk'
[ https://svn.apache.org/r1678535 ]
SOLR-7522: single valued numeric field faceting
Commit 1678536 from Yonik Seeley in branch 'dev/branches/branch_5x'
[ https://svn.apache.org/r1678536 ]
Bulk close for 5.2.0.