fix: flatten ValueTypeSlice to ValueTypeString

[crossk3]

:wave:

While working with the OpenTelemetry stack for Python as well as an OpenTelemetry Collector (Cloud Run Sidecar, per docs here), I noticed a discrepancy in behavior.

GCP Cloud Trace doesn't support Slice/Array data types for Trace Attributes, which Python's CloudTraceSpanExporter handles by flattening Slices to Strings. However, the golang Collector Exporter currently silently drops any Slice[T] attribute. This results in some oddities when choosing how to send traces to GCP Cloud Trace, where any time a collector exports to GCP Cloud Trace you silently lose attributes you would otherwise receive in Cloud Trace.

This PR attempts to unify this behavior, so that the same behavior is observed whether using the in-process exporter or the googlecloud Collector exporter.

[dashpole]

Thanks for finding this @crossk3!

[crossk3]

@dashpole updated to use json.Marshal :slightly_smiling_face:

[dashpole]

/gcbrun

[dashpole]

/gcbrun

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 62.30%. Comparing base (4caace7) to head (542da19). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #831 +/- ## ========================================== + Coverage 61.03% 62.30% +1.26% ========================================== Files 56 56 Lines 5903 4892 -1011 ========================================== - Hits 3603 3048 -555 + Misses 2143 1687 -456 Partials 157 157 ```

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[dashpole]

Ah, you will need to run make fixtures to regenerate the test files.

[crossk3]

Ah, you will need to run make fixtures to regenerate the test files.

@dashpole done!

[dashpole]

/gcbrun