filterCache ramBytesUsed monitoring statistics go negative

[Dawn]

Hi:

        filterCache ramBytesUsed monitoring statistics go negative.

        Is there a special meaning, or is there a statistical problem
 
        When present the list, can sort it by key. Solr7 is like this, easy to 
view.


For example:

CACHE.searcher.filterCache.hits:
    63265

CACHE.searcher.filterCache.cumulative_evictions:
    1981

CACHE.searcher.filterCache.size:
    6765

CACHE.searcher.filterCache.maxRamMB:
    10240

CACHE.searcher.filterCache.hitratio:
    0.8329712577846243

CACHE.searcher.filterCache.warmupTime:
    49227

CACHE.searcher.filterCache.evictions:
    1981

CACHE.searcher.filterCache.cumulative_hitratio:
    0.737519464195261

CACHE.searcher.filterCache.lookups:
    75951

CACHE.searcher.filterCache.cumulative_hits:
    78624

CACHE.searcher.filterCache.cumulative_inserts:
    15927

CACHE.searcher.filterCache.ramBytesUsed:
    -1418740612

CACHE.searcher.filterCache.inserts:
    10510

CACHE.searcher.filterCache.cumulative_lookups:
    106606