Refactor Gauge metrics to be homeserver-scoped (#18725)

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

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 TODO metrics with the `server_name`
label

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

tests/server.py

@@ -702,6 +702,7 @@ def make_fake_db_pool(
     reactor: ISynapseReactor,
     db_config: DatabaseConnectionConfig,
     engine: BaseDatabaseEngine,
+    server_name: str,
 ) -> adbapi.ConnectionPool:
     """Wrapper for `make_pool` which builds a pool which runs db queries synchronously.
 
@@ -710,7 +711,9 @@ def make_fake_db_pool(
     is a drop-in replacement for the normal `make_pool` which builds such a connection
     pool.
     """
-    pool = make_pool(reactor, db_config, engine)
+    pool = make_pool(
+        reactor=reactor, db_config=db_config, engine=engine, server_name=server_name
+    )
 
     def runWithConnection(
         func: Callable[..., R], *args: Any, **kwargs: Any