Update debug log to be post processors (#2932)

I was looking for _sampling_priority_v1 attributes in debug log and they were missing, checked with Kyle and it is indeed the [update_tags processor call that sets it](https://github.com/DataDog/dd-trace-py/blob/fcecc4748d811647159caaed2aa05cd68c0f5269/ddtrace/internal/processor/trace.py#L85)

(cherry picked from commit 1dd2942)

Co-authored-by: Raphael Gavache <[email protected]>

Author: mergify[bot]

ddtrace/tracer.py

@@ -638,9 +638,6 @@ def _start_span(
 
     def _on_span_finish(self, span):
         # type: (Span) -> None
-        if self.log.isEnabledFor(logging.DEBUG):
-            self.log.debug("finishing span %s (enabled:%s)", span.pprint(), self.enabled)
-
         active = self.current_span()
         # Debug check: if the finishing span has a parent and its parent
         # is not the next active span then this is an error in synchronous tracing.
@@ -655,6 +652,9 @@ def _on_span_finish(self, span):
         for p in self._span_processors:
             p.on_span_finish(span)
 
+        if self.log.isEnabledFor(logging.DEBUG):
+            self.log.debug("finishing span %s (enabled:%s)", span.pprint(), self.enabled)
+
     def _initialize_span_processors(self):
         # type: () -> None
         trace_processors = []  # type: List[TraceProcessor]