Refactor `GaugeBucketCollector` metrics to be homeserver-scoped
Part of https://github.com/element-hq/synapse/issues/18592
### Testing strategy
1. Add the `metrics` listener in your `homeserver.yaml`
```yaml
listeners:
# This is just showing how to configure metrics either way
#
# `http` `metrics` resource
- port: 9322
type: http
bind_addresses: ['127.0.0.1']
resources:
- names: [metrics]
compress: false
# `metrics` listener
- port: 9323
type: metrics
bind_addresses: ['127.0.0.1']
```
1. Start the homeserver: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Fetch `http://localhost:9322/_synapse/metrics` and/or
`http://localhost:9323/metrics`
1. Adjust the number of [`msecs` in the `looping_call` so that
`_read_forward_extremities`](https://github.com/element-hq/synapse/blob/a82b8a966a7dbe218cb788548683a83ec404e468/synapse/storage/databases/main/metrics.py#L79)
runs immediately instead of after an hour.
1. Observe response includes the `synapse_forward_extremities` and
`synapse_excess_extremity_events` metrics with the `server_name` label
Eric Eastwood
5106818bd0