ovsdb-server: fix monitor counters reported by memory/show

ovsdb-server: fix monitor counters reported by memory/show

Commit 4c2809787cdbc774 (ovsdb-monitor: add json cache) introduced
an optimization that allows jsonrpc session to share monitors.
However, the memory/show implementation was not updated to match the
implementation; it still assumes that each jsonrpc session uses its
own monitor, thus are likely to over reporting the number.

This patch fix the bug and reports the actual number of monitor used
by the ovsdb-server.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>