Flatten field + fix u64 to string serde

[hwrdtm]

Changes

This PR:

• Is a continuation of what was originally surfaced Fix serde #1856.
• Flattens some data and value fields in order to be consistent with the JSON schema implemented on the OTEL Collector
  • See here for the struct defs and JSON tags
  • Also see here for an example resource metric in JSON
• Adds more fields to be correctly doing serde between u64 and String
• Without both of these changes the ./expected/metrics.json file wouldn't even deserialize correctly.

Testing

• Run cargo test --workspace --all-features --test grpc_build -- --nocapture to generate grpc files. Running the first time is expected to fail, as it will create the files.
• Run cargo test --workspace --all-features --test metrics 'test_serde' -- --nocapture to run the new test case introduced in this PR.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

cc @lalitb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.9%. Comparing base (cdf5f03) to head (4a92519).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1911   +/-   ##
=====================================
  Coverage   74.9%   74.9%           
=====================================
  Files        122     122           
  Lines      20308   20308           
=====================================
  Hits       15217   15217           
  Misses      5091    5091           

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[hwrdtm]

Any thoughts here?

[lalitb]

Thanks for the PR @hwrdtm . Will review it tomorrow.

[TommyCpp]

We can use tempfile here