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))

scripts-dev/mypy_synapse_plugin.py

@@ -28,7 +28,7 @@ from typing import Callable, Optional, Tuple, Type, Union
 import mypy.types
 from mypy.erasetype import remove_instance_last_known_values
 from mypy.errorcodes import ErrorCode
-from mypy.nodes import ARG_NAMED_OPT, ListExpr, NameExpr, TempNode, Var
+from mypy.nodes import ARG_NAMED_OPT, ListExpr, NameExpr, TempNode, TupleExpr, Var
 from mypy.plugin import (
     FunctionLike,
     FunctionSigContext,
@@ -61,6 +61,7 @@ class SynapsePlugin(Plugin):
     ) -> Optional[Callable[[FunctionSigContext], FunctionLike]]:
         if fullname in (
             "prometheus_client.metrics.Counter",
+            "prometheus_client.metrics.Gauge",
             # TODO: Add other prometheus_client metrics that need checking as we
             # refactor, see https://github.com/element-hq/synapse/issues/18592
         ):
@@ -98,8 +99,8 @@ def check_prometheus_metric_instantiation(ctx: FunctionSigContext) -> CallableTy
     ensures metrics are correctly separated by homeserver.
 
     There are also some metrics that apply at the process level, such as CPU usage,
-    Python garbage collection, Twisted reactor tick time which shouldn't have the
-    `SERVER_NAME_LABEL`. In those cases, use use a type ignore comment to disable the
+    Python garbage collection, and Twisted reactor tick time, which shouldn't have the
+    `SERVER_NAME_LABEL`. In those cases, use a type ignore comment to disable the
     check, e.g. `# type: ignore[missing-server-name-label]`.
     """
     # The true signature, this isn't being modified so this is what will be returned.
@@ -136,7 +137,7 @@ def check_prometheus_metric_instantiation(ctx: FunctionSigContext) -> CallableTy
     # ]
     # ```
     labelnames_arg_expression = ctx.args[2][0] if len(ctx.args[2]) > 0 else None
-    if isinstance(labelnames_arg_expression, ListExpr):
+    if isinstance(labelnames_arg_expression, (ListExpr, TupleExpr)):
         # Check if the `labelnames` argument includes the `server_name` label (`SERVER_NAME_LABEL`).
         for labelname_expression in labelnames_arg_expression.items:
             if (