Review the metrics API to ensure consistent placeholders for missing values

Spin-off from SOLR-14657. Some gauges can legitimately be missing or in an unknown state at some points in time, eg. during SolrCore startup or shutdown.

Currently the API returns placeholders with either impossible values for numeric gauges (such as index size -1) or empty maps / strings for other non-numeric gauges.

hossman noticed that the values for these placeholders may be misleading, depending on how the user treats them - if the client has no special logic to treat them as "missing values" it may erroneously treat them as valid data. E.g. numeric values of -1 or 0 may severely skew averages and produce misleading peaks / valleys in metrics histories.

On the other hand returning a literal null value instead of the expected number may also cause unexpected client issues - although in this case it's clearer that there's actually no data available, so long-term this may be a better strategy than returning impossible values, even if it means that the client should learn to handle null values appropriately.