Add ability to spawn child Celery tasks that do NOT inherit the parent's trace id

[RaduW]

Problem Statement

We would like to be able to spawn Celery tasks that do not have the parent's task trace_id .

The reason for this is that sometimes it makes sense to sample the child tasks independently from the parent task ( on the server, i.e. dynamic sampling).

A typical scenario is of a task that runs seldom (once a week) and that spawns a lot of independent tasks ( e.g. one task per organisation). Having the child tasks linked by trace_id would mean that we either keep all traces from all organisations (tens of thousands) or none when we would actually need to always keep the traces for a few orgs.

Currently we can disable passing trace_id but only globally for all tasks and all errors.
This is not a usable solution since a lot of times passing the trace_id makes sense.

Solution Brainstorm

There is already a proof of concept PR opened by @jan-auer that implements the feature ( no tests though)
2331

In this proof of concept, we are passing a header in apply_async() that only disables passing the trace_id for the current call.

[antonpirker]

Fixed here: #2331