Refactor Measure block metrics to be homeserver-scoped (v2) (#18601)

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)

tests/handlers/test_typing.py

@@ -86,6 +86,7 @@ class TypingNotificationsTestCase(unittest.HomeserverTestCase):
         self.mock_federation_client = AsyncMock(spec=["put_json"])
         self.mock_federation_client.put_json.return_value = (200, "OK")
         self.mock_federation_client.agent = MatrixFederationAgent(
+            "OUR_STUB_HOMESERVER_NAME",
             reactor,
             tls_client_options_factory=None,
             user_agent=b"SynapseInTrialTest/0.0.0",