Improve opentracing for federation requests (#11870)

The idea here is to set the parent span for incoming federation requests to the
*outgoing* span on the other end. That means that you can see (most of) the
full end-to-end flow when you have a process that includes federation requests.

However, in order not to lose information, we still want a link to the
`incoming-federation-request` span from the servlet, so we have to create
another span to do exactly that.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/11870.misc b/changelog.d/11870.misc
new file mode 100644
index 0000000000..2cb0efdb45
--- /dev/null
+++ b/changelog.d/11870.misc
@@ -0,0 +1 @@
+Improve opentracing for incoming federation requests.