OSD: mark two heartbeat config opts as observed

"osd heartbeat min size" and "osd heartbeat interval" can be changed
at runtime, because their values, when used, are always taken from
global Ceph configuration. Mark them as observed, so the message
the user sees once they're changed doesn't confuse them.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>
(cherry picked from commit 44b7839)

src/osd/OSD.cc

@@ -9020,6 +9020,8 @@ const char** OSD::get_tracked_conf_keys() const
     "fsid",
     "osd_client_message_size_cap",
     "osd_client_message_cap",
+    "osd_heartbeat_min_size",
+    "osd_heartbeat_interval",
     NULL
   };
   return KEYS;