fix(js): process spans in isolation to improve accuracy of telemetry

[andrewbrook]

Span processing in the google-cloud plugin is currently done in batches, which are chosen by the OTel libraries. We erroneously rely on each batch being a complete representation of a trace, and to contain only spans from a single trace. Since these assumptions are incorrect there are currently subtle bugs where span metrics and logs are being mis-attributed, or dropped. This change removes the need for the above assumptions, by processing each span entirely independently.

We also remove some unused logs/metrics for simplicity, and improve the test coverage around "path" telemetry.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)

[schnecle]

This looks great - the simplification is so good! Thanks Andrew!