Fix `LaterGauge` metrics to collect from all servers
Follow-up to https://github.com/element-hq/synapse/pull/18714
Previously, our `LaterGauge` metrics did include the `server_name` label
as expected but we were only seeing the last server being reported in
some cases. Any `LaterGauge` that we were creating multiple times was
only reporting the last instance.
This PR updates all `LaterGauge` to be created once and then we use
`LaterGauge.register_hook(...)` to add in the metric callback as before.
This works now because we store a list of callbacks instead of just one.
I noticed this problem thanks to some [tests in the Synapse Pro for
Small Hosts](https://github.com/element-hq/synapse-small-hosts/pull/173)
repo that sanity check all metrics to ensure that we can see each metric
includes data from multiple servers.
### Testing strategy
1. This is only noticeable when you run multiple Synapse instances in
the same process.
1. TODO
(see test that was added)
### Dev notes
Previous non-global `LaterGauge`:
```
synapse_federation_send_queue_xxx
synapse_federation_transaction_queue_pending_destinations
synapse_federation_transaction_queue_pending_pdus
synapse_federation_transaction_queue_pending_edus
synapse_handlers_presence_user_to_current_state_size
synapse_handlers_presence_wheel_timer_size
synapse_notifier_listeners
synapse_notifier_rooms
synapse_notifier_users
synapse_replication_tcp_resource_total_connections
synapse_replication_tcp_command_queue
synapse_background_update_status
synapse_federation_known_servers
synapse_scheduler_running_tasks
```
### Pull Request Checklist
<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->
* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
076db0ab49