Fixes for opentracing scopes (#11869)

`start_active_span` was inconsistent as to whether it would activate the span
immediately, or wait for `scope.__enter__` to happen (it depended on whether
the current logcontext already had an associated scope). The inconsistency was
rather confusing if you were hoping to set up a couple of separate spans before
activating either.

Looking at the other implementations of opentracing `ScopeManager`s, the
intention is that it *should* be activated immediately, as the name
implies. Indeed, the idea is that you don't have to use the scope as a
contextmanager at all - you can just call `.close` on the result. Hence, our
cleanup has to happen in `.close` rather than `.__exit__`.

So, the main change here is to ensure that `start_active_span` does activate
the span, and that `scope.close()` does close the scope.

We also add some tests, which requires a `tracer` param so that we don't have
to rely on the global variable in unit tests.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/11869.misc b/changelog.d/11869.misc
new file mode 100644
index 0000000000..054fbf6101
--- /dev/null
+++ b/changelog.d/11869.misc
@@ -0,0 +1 @@
+Ensure that `opentracing` scopes are activated and closed at the right time.