Do not include fields with nil values in datapoint

[bszaf]

We may have situation like:

Time    X       Y       Z
  1 .   nil     nil      0
  2 .    1      nil      0
  3 .    1      nil      0

So in this situation we want to get rid of all Y values and X value for 1st metric. In library representation it would look like this:

%{"name" => "some_measurement",
    "data" => [
      %{"Z" => 0, "time" => 1},
      %{"X" => 1, "Z" => 0, "time" => 2},
      %{"X" => 1, "Z" => 0, "time" => 3},
   ]
}