Don't duplicate the metric options.

[kape1395]

Before this change, the options were duplicated:

1> exometer:ensure([a], gauge, [{description, "very nice"}]).
2> exometer:info([a]).
[{name,[a]},
 {type,gauge},
 {behaviour,undefined},
 {module,exometer},
 {status,enabled},
 {cache,0},
 {value,[{value,0},{ms_since_reset,13018}]},
 {timestamp,466310610978},
 {options,[{description,"very nice"},
           {description,"very nice"}]},
 {ref,undefined}]