doc: include read, write, metadata average latencies in doc/man.

Also, the sample cephfs-top image in the doc is outdated. Update that!

Fixes: http://tracker.ceph.com/issues/48619
Signed-off-by: Venky Shankar <vshankar@redhat.com>

doc/cephfs/cephfs-top.rst

@@ -29,7 +29,7 @@ metrics are for a particular MDS rank (e.g., number of subtrees handled by an MD
 Once enabled, Ceph Filesystem metrics can be fetched via::
 
   $ ceph fs perf stats
-  {"version": 1, "global_counters": ["cap_hit", "read_latency", "write_latency", "metadata_latency", "dentry_lease"], "counters": [], "client_metadata": {"client.614146": {"IP": "10.1.1.100", "hostname"  : "ceph-host1", "root": "/", "mount_point": "/mnt/cephfs", "valid_metrics": ["cap_hit", "read_latency", "write_latency", "metadata_latency", "dentry_lease"]}}, "global_metrics": {"client.614146": [[0,  0], [0, 0], [0, 0], [0, 0], [0, 0]]}, "metrics": {"delayed_ranks": [], "mds.0": {"client.614146": []}}}
+  {"version": 1, "global_counters": ["cap_hit", "read_latency", "write_latency", "metadata_latency", "dentry_lease", "opened_files", "pinned_icaps", "opened_inodes", "avg_read_latency", "stdev_read_latency", "avg_write_latency", "stdev_write_latency", "avg_metadata_latency", "stdev_metadata_latency"], "counters": [], "client_metadata": {"client.324130": {"IP": "192.168.1.100", "hostname": "ceph-host1", "root": "/", "mount_point": "/mnt/cephfs", "valid_metrics": ["cap_hit", "read_latency", "write_latency", "metadata_latency", "dentry_lease, "opened_files", "pinned_icaps", "opened_inodes", "avg_read_latency", "stdev_read_latency", "avg_write_latency", "stdev_write_latency", "avg_metadata_latency", "stdev_metadata_latency"]}}, "global_metrics": {"client.324130": [[309785, 1280], [0, 0], [197, 519015022], [88, 279074768], [12, 70147], [0, 3], [3, 3], [0, 3], [0, 0], [0, 0], [0, 11699223], [0, 88245], [0, 6596951], [0, 9539]]}, "metrics": {"delayed_ranks": [], "mds.0": {"client.324130": []}}}
 
 Details of the JSON command output are as follows:
 

doc/man/8/cephfs-top.rst

@@ -43,18 +43,6 @@ Descriptions of fields
 
    cap hit rate
 
-.. describe:: rlat
-
-   read latency
-
-.. describe:: wlat
-
-   write latency
-
-.. describe:: mlat
-
-   metadata latency
-
 .. describe:: dlease
 
    dentry lease rate
@@ -95,6 +83,29 @@ Descriptions of fields
 
    speed of write IOs compared with the last refresh
 
+.. describe:: rlatavg
+
+   average read latency
+
+.. describe:: rlatsd
+
+   standard deviation (variance) for read latency
+
+.. describe:: wlatavg
+
+   average write latency
+
+.. describe:: wlatsd
+
+   standard deviation (variance) for write latency
+
+.. describe:: mlatavg
+
+   average metadata latency
+
+.. describe:: mlatsd
+
+   standard deviation (variance) for metadata latency
 
 Availability
 ============