OTel: Task Processor

[khvn26]

When a Django view enqueues a task, the OTel trace context (TraceID, SpanID, baggage) is available. By the time the task processor picks it up, that context is gone. This means:

• Task spans can't be linked to the original request trace
• structlog events emitted during task execution have empty TraceID/SpanID
• W3C baggage (e.g. amplitude.device_id, amplitude.session_id) is lost

We want to propagate OTel trace context through task serialisation using OTel's propagate.inject / propagate.extract API.

Add a trace_context JSONField (nullable, default empty dict) to AbstractBaseTask. Requires a migration.

When creating a Task, inject the current OTel context into a carrier dict and store it alongside the task:

from opentelemetry import propagate

carrier: dict[str, str] = {}
propagate.inject(carrier)
# Store carrier in task.trace_context field

In _run_task(), extract the context and activate it before calling the task function:

from opentelemetry import context, propagate, trace

ctx = propagate.extract(task.trace_context)
tracer = trace.get_tracer("task_processor")
with tracer.start_as_current_span(task.task_identifier, context=ctx):
    task.run()