Fix Failed to stop metrics warnings in request metrics (#18753)

```
Failed to stop metrics: TypeError("prometheus_client.metrics.MetricWrapperBase.labels() got multiple values for keyword argument 'server_name'")
```

Noticed while running and debugging some tests.

This bug was introduced in
#18724

Author: MadLittleMods

changelog.d/18753.misc

@@ -0,0 +1 @@
+Refactor `Histogram` metrics to be homeserver-scoped.

synapse/http/request_metrics.py

@@ -243,7 +243,6 @@ def stop(self, time_sec: float, response_code: int, sent_bytes: int) -> None:
         response_timer.labels(
             code=response_code_str,
             **response_base_labels,
-            **{SERVER_NAME_LABEL: self.our_server_name},
         ).observe(time_sec - self.start_ts)
 
         resource_usage = context.get_resource_usage()