fix(tracing): report spans whose parent lives on another node

[solnic]

When a service continued a trace from a traceparent and its local root was anything other than an HTTP-server / LiveView / Oban-consumer span etc., process_span/1 fell through to the server_span?/1 heuristic and the entire local subtree was silently dropped.

SpanRecord already carries OTel's parent_span_is_remote, so the local root of remotely-continued trace can be identified directly and reported as its own segment. build_trace_context/2 already passes parent_span_id through, so it nests under the upstream transaction.

This works on every supported opentelemetry version.

⚠️ parent_span_is_remote only exists on the span record from opentelemetry >= 1.6, so on 1.5.x (what Elixir 1.13/1.14 lock) the span processor derives it from the parent span context at on_start and carries it via an internal attribute that never reaches a payload.

Before

After

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Fixes #1166