Reduce volume of metrics retrieved on MySQL Command/Handler Compare dashboard

The default of All creates a lot of queries and load on PMM Server, and likely not many of the graphs get looked at.  Here is a pattern that should return non zero values to print interesting lines:

var-command=alter_table&var-command=delete&var-command=insert&var-command=replace&var-command=select&var-command=update&var-handler=commit&var-handler=delete&var-handler=update&var-handler=write

https://pmmdemo.percona.com/graph/d/000000170/mysql-command-handler-counters-compare?orgId=1&var-interval=$__auto_interval_interval&var-host=mdb102&var-command=alter_table&var-command=delete&var-command=insert&var-command=replace&var-command=select&var-command=update&var-handler=commit&var-handler=delete&var-handler=update&var-handler=write

TODO: We should specify Default values to Variables in Dashboard to use this ^^ set of values