Metrics work through http listener when enable_metrics false

This issue has been migrated from [#16222](https://github.com/matrix-org/synapse/issues/16222).

---

### Description

Not a critical bug, but the "http" listener on generic_workers allows metrics when `enable_metrics: false` - this doesn't happen on the Synapse main process, which informs the user that metrics are disabled until `enable_metrics` is changed.

### Steps to reproduce

- Set `enable_metrics: false` in homeserver.yaml
- Create any generic_worker
- Create worker_listener of "http" type with `["metrics"]` resource
- Metrics are now served through that listener

### Homeserver

tcpip.uk

### Synapse Version

1.91.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

PostgreSQL 15.4

### Workers

Multiple workers

### Platform

Ubuntu 22.04 baremetal

### Configuration

_No response_

### Relevant log output

Not sure what to put here, metrics are served (through Twisted rather than sidecar) when the Synapse main process doesn't allow it.

### Anything else that would be useful to know?

_No response_