Update REFERENCE.md to include collector.dbstatsfreestorage (#791)

* Update REFERENCE.md

To include the --collector.dbstatsfreestorage option introduced in v.0.40.0.

* Update REFERENCE.md

Co-authored-by: Michael Okoko <[email protected]>

---------

Co-authored-by: Jiří Čtvrtka <[email protected]>
Co-authored-by: Nurlan Moldomurov <[email protected]>
Co-authored-by: Michael Okoko <[email protected]>
Co-authored-by: Nurlan Moldomurov <[email protected]>

Author: 4 people

REFERENCE.md

@@ -19,6 +19,7 @@
 |--collector.diagnosticdata|Enable collecting metrics from getDiagnosticData|
 |--collector.replicasetstatus|Enable collecting metrics from replSetGetStatus|
 |--collector.dbstats|Enable collecting metrics from dbStats||
+|--collector.dbstatsfreestorage|Enable collecting freeStorage metrics from dbStats. If the instance has a large number of collections or indexes, obtaining free space usage data may cause processing delays||
 |--collector.topmetrics|Enable collecting metrics from top admin command|
 |--collector.currentopmetrics|Enable collecting metrics from currentop admin command|
 |--collector.indexstats|Enable collecting metrics from $indexStats|