Add span activate and deactivate apis

MIGRATION_GUIDE.md

@@ -168,7 +168,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
   - `span_id`
   - `parent_span_id`: you can supply a `parent_span` instead
 - The `Scope.transaction` property has been removed. To obtain the root span (previously transaction), use `Scope.root_span`. To set the root span's (transaction's) name, use `Scope.set_transaction_name()`.
-- The `Scope.span =` setter has been removed.
+- The `Scope.span =` setter has been removed. Please use the new `span.activate()` api instead if you want to activate a new span manually instead of using the `start_span` context manager.
 - Passing a list or `None` for `failed_request_status_codes` in the Starlette integration is no longer supported. Pass a set of integers instead.
 - The `span` argument of `Scope.trace_propagation_meta` is no longer supported.
 - Setting `Scope.user` directly is no longer supported. Use `Scope.set_user()` instead.

sentry_sdk/tracing.py

@@ -270,15 +270,21 @@ def __repr__(self):
             )
         )
 
-    def __enter__(self):
-        # type: () -> Span
-        # XXX use_span? https://github.com/open-telemetry/opentelemetry-python/blob/3836da8543ce9751051e38a110c0468724042e62/opentelemetry-api/src/opentelemetry/trace/__init__.py#L547
-        #
-        # create a Context object with parent set as current span
+    def activate(self):
+        # type: () -> None
         ctx = otel_trace.set_span_in_context(self._otel_span)
         # set as the implicit current context
         self._ctx_token = context.attach(ctx)
 
+    def deactivate(self):
+        # type: () -> None
+        if self._ctx_token:
+            context.detach(self._ctx_token)
+            del self._ctx_token
+
+    def __enter__(self):
+        # type: () -> Span
+        self.activate()
         return self
 
     def __exit__(self, ty, value, tb):
@@ -294,8 +300,7 @@ def __exit__(self, ty, value, tb):
                 self.set_status(SPANSTATUS.OK)
 
         self.finish()
-        context.detach(self._ctx_token)
-        del self._ctx_token
+        self.deactivate()
 
     @property
     def description(self):