Add metrics to track how often events are `soft_failed` (#10156)

Spawned from missing messages we were seeing on `matrix.org` from a
federated Gtiter bridged room, https://gitlab.com/gitterHQ/webapp/-/issues/2770.
The underlying issue in Synapse is tracked by https://github.com/matrix-org/synapse/issues/10066
where the message and join event race and the message is `soft_failed` before the
`join` event reaches the remote federated server.

Less soft_failed events = better and usually this should only trigger for events
where people are doing bad things and trying to fuzz and fake everything.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/10156.misc b/changelog.d/10156.misc
new file mode 100644
index 0000000000..92a188b87b
--- /dev/null
+++ b/changelog.d/10156.misc
@@ -0,0 +1 @@
+Add `synapse_federation_soft_failed_events_total` metric to track how often events are soft failed.