cloudv2: Not replicate common fields

[codebien]

What?

It moves the repeated fields in a shared place and then the backend will denormalize them.

Why?

It reduces memory allocation on k6 open source.

Related PR(s)/Issue(s)

Updates #3117

[codecov-commenter]

Codecov Report

Merging #3144 (c3749c7) into master (1df72a0) will decrease coverage by 0.01%.
The diff coverage is 40.00%.

❗ Current head c3749c7 differs from pull request most recent head c38bcd7. Consider uploading reports for the commit c38bcd7 to get more accurate results

@@            Coverage Diff             @@
##           master    #3144      +/-   ##
==========================================
- Coverage   71.98%   71.98%   -0.01%     
==========================================
  Files         252      252              
  Lines       19405    19403       -2     
==========================================
- Hits        13969    13967       -2     
- Misses       4542     4543       +1     
+ Partials      894      893       -1     

Flag	Coverage Δ
ubuntu	71.91% <40.00%> (+<0.01%)	⬆️
windows	71.83% <40.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
output/cloud/expv2/hdr.go	95.71% <ø> (-0.12%)	⬇️
output/cloud/expv2/pbcloud/metric.pb.go	8.42% <0.00%> (-0.06%)	⬇️
output/cloud/expv2/flush.go	86.95% <100.00%> (ø)
output/cloud/expv2/mapping.go	42.64% <100.00%> (-2.43%)	⬇️

... and 2 files with indirect coverage changes

[codebien]

I still need to iterate on #3144 (comment) but we can do reviews in async considering how small the impact should be.

[imiric]

LGTM, just have a naming suggestion.

Also, isn't this technically a breaking change for the backend, since they will now read the test run ID and aggregation period from the top MetricSet object? Have you coordinated this with them?