Fix potential nil pointer dereferences in kubelet-to-gcm

[courageJ]

This PR hardens the kubelet-to-gcm monitoring daemon against crashes by adding nil pointer protections across several components.

Changes

• Translation Logic: Added nil checks for optional Kubelet stats fields (UsedBytes in FsStats, MajorPageFaults in MemoryStats) before dereferencing them in monitor/kubelet/translate.go.
• Service & Client Safety: Added nil checks for the GCM service pointer in the Once function (monitor/poll.go) and for HTTP responses in client helpers (monitor/controller/client.go and monitor/kubelet/client.go) to prevent panics during defer response.Body.Close().
• Helpers: Added Uint64Ptr to the monitor package for parity with existing pointer helpers.
• Testing: Added a TestTranslator_NilFields regression test in monitor/kubelet/translate_test.go to verify that missing optional fields in stats.Summary do not cause panics.

Verification

• All existing unit tests passed.
• New regression test TestTranslator_NilFields passed and verified the fixes.