parametric: avoids sending non-null default values to trace/otel/start_span

[mabdinur]

Motivation

• Currently the trace/otel/start_span endpoint receives a default start timestamp of 0, default spankind of unimplemented, and a default parent_id of 0. These default values add unnecessary complexity to parametric endpoints.

Changes

• Updates the default value of:
  • timestamp from 0 to None
  • span_kind from unimplemented to None
  • parent_id from 0 to None
• Updates parametric apps to ignore null/unset arguments.

Workflow

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Reviewer checklist

• If PR title starts with [<language>], double-check that only <language> is impacted by the change
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