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))

docs/upgrade.md

@@ -117,6 +117,25 @@ each upgrade are complete before moving on to the next upgrade, to avoid
 stacking them up. You can monitor the currently running background updates with
 [the Admin API](usage/administration/admin_api/background_updates.html#status).
 
+# Upgrading to v1.136.0
+
+## Metric labels have changed on `synapse_federation_last_received_pdu_time` and `synapse_federation_last_sent_pdu_time`
+
+Previously, the `synapse_federation_last_received_pdu_time` and
+`synapse_federation_last_sent_pdu_time` metrics both used the `server_name` label to
+differentiate between different servers that we send and receive events from.
+
+Since we're now using the `server_name` label to differentiate between different Synapse
+homeserver instances running in the same process, these metrics have been changed as follows:
+
+ - `synapse_federation_last_received_pdu_time` now uses the `origin_server_name` label
+ - `synapse_federation_last_sent_pdu_time` now uses the `destination_server_name` label
+
+The Grafana dashboard JSON in `contrib/grafana/synapse.json` has been updated to reflect
+this change but you will need to manually update your own existing Grafana dashboards
+using these metrics.
+
+
 # Upgrading to v1.135.0
 
 ## `on_user_registration` module API callback may now run on any worker