Corrected the dashboard to use the new metrics
The metric seems to have changed from SeaweedFS_filer_ to SeaweedFS_filerStore_. This commit replaces the ref in the dashboard with the new series name.
In the current dashboard there is no way to know the status of
volume servers, as prometheus only accepts push information, if
any server volume goes offline, the last information sent will
be read continuously by grafana.
Added a graph that calculates the delta between the information
of the last push timestamp with the timestamp of 15 seconds ago,
if the timestamp has not been changed, server state will be 0,
if volume server sent any information the server state will be 1.
-This graph will have to be modified if the time interval of pushs
is modified, a suggestion for improvement is to send the interval
in seconds along with the metrics.
-Only local tests were run, maybe if the latency between pushgateway
and volume server is too high, the delta calculation could be
affected.
