Add more detailed telemetry for get_audience

This is still slow in some cases, despite #2723, so this information
should give useful data about how it could be optimized more.

This also adds some abstraction around getting the tracer, just to
follow the advice in the OpenTelemetry documentation not to use __name__
directly to set the tracer name. The advice is ignored in most of their
examples, so it probably doesn't matter, but IDK, seems reasonable to
try to follow it.

Related: #2720

bookwyrm/telemetry/open_telemetry.py

@@ -29,3 +29,7 @@ def instrumentCelery():
     @worker_process_init.connect(weak=False)
     def init_celery_tracing(*args, **kwargs):
         CeleryInstrumentor().instrument()
+
+
+def tracer():
+    return trace.get_tracer(__name__)