Refactor `Measure` block metrics to be homeserver-scoped (add
`server_name` label to block metrics).
Part of https://github.com/element-hq/synapse/issues/18592
### Testing strategy
#### See behavior of previous `metrics` listener
1. Add the `metrics` listener in your `homeserver.yaml`
```yaml
listeners:
- 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:9323/metrics`
1. Observe response includes the block metrics
(`synapse_util_metrics_block_count`,
`synapse_util_metrics_block_in_flight`, etc)
#### See behavior of the `http` `metrics` resource
1. Add the `metrics` resource to a new or existing `http` listeners in
your `homeserver.yaml`
```yaml
listeners:
- port: 9322
type: http
bind_addresses: ['127.0.0.1']
resources:
- names: [metrics]
compress: false
```
1. Start the homeserver: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Fetch `http://localhost:9322/_synapse/metrics` (it's just a `GET`
request so you can even do in the browser)
1. Observe response includes the block metrics
(`synapse_util_metrics_block_count`,
`synapse_util_metrics_block_in_flight`, etc)
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
