Dockbeat cannot collect memory stats on Windows

[godefroi]

Docker on Windows reports different memory statistics than it does on Linux. Because some expected values are not present, the JSON serialization fails, reporting that NaN is an invalid value.

On Windows, the memory statistics output by Docker look like this:

"memory_stats":{  
      "commitbytes":62787584,
      "commitpeakbytes":95756288,
      "privateworkingset":41222144
   }