changed ContainerNetworkStat to a pointer

[JVecsei1]

• Related to Apps network usage metrics missing cf-networking-release#64
• Changed ContainerNetworkStat to a pointer to differentiate between empty and zero values
• Required for added container network metrics guardian#417 (comment) and added container network metrics guardian#417 (comment)
• Allows a backwards compatible implementation and avoids sending non-existent metrics for the windows implementation in the executor.

[cf-gitbot]

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