Part of https://github.com/element-hq/synapse/issues/18592
Separated out of https://github.com/element-hq/synapse/pull/18656
because it's a bigger, unique piece of the refactor
### 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. Observe response includes the background processs metrics
(`synapse_background_process_start_count`,
`synapse_background_process_db_txn_count_total`, etc) with the
`server_name` label
b7e7f537f1