Refactor Counter metrics to be homeserver-scoped (#18656)

Bulk refactor `Counter` metrics to be homeserver-scoped. We also add
lints to make sure that new `Counter` metrics don't sneak in without
using the `server_name` label (`SERVER_NAME_LABEL`).

All of the "Fill in" commits are just bulk refactor.

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. Observe response includes the `synapse_user_registrations_total`,
`synapse_http_server_response_count_total`, etc metrics with the
`server_name` label

tests/server.py

@@ -432,7 +432,7 @@ def make_request(
 
     channel = FakeChannel(site, reactor, ip=client_ip)
 
-    req = request(channel, site)
+    req = request(channel, site, our_server_name="test_server")
     channel.request = req
 
     req.content = BytesIO(content)