Add json tags on cloudinit Metadata

[yitsushi]

What this PR does / why we need it:

Add json tags on the Metadata struct, so when we marshal it, it uses the same naming as the yaml format (and the one we are using right now as a key-value string).

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes

[codecov-commenter]

Codecov Report

Merging #319 (b8d0ab1) into main (7213b9c) will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
- Coverage   58.15%   57.95%   -0.20%     
==========================================
  Files          51       51              
  Lines        2509     2509              
==========================================
- Hits         1459     1454       -5     
- Misses        935      938       +3     
- Partials      115      117       +2     

Impacted Files	Coverage Δ
pkg/queue/queue.go	93.93% <0.00%> (-6.07%)	⬇️
infrastructure/containerd/event_service.go	56.14% <0.00%> (-3.51%)	⬇️
pkg/process/process.go	37.73% <0.00%> (-1.89%)	⬇️

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