Fastify etag bringing QPS down for paths where dummy etags have been added

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

4.15.0

### Plugin version

4.2.0

### Node.js version

16.5.1

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

12.6.5

### Description

I used the fastify etag plugin and for the url paths where I did not require etag I added a dummy etag but the QPS for both the paths decreased significantly after adding the plugin

I also tried generating etag myself and this seemed to be give me better QPS. Any reasons for this strange behaviour or is this expected?

```js
const hash = crypto.createHash('sha256');
const stringDoc: string = JSON.stringify(documentData);
etag = hash.update(stringDoc).digest('hex');
```

### Steps to Reproduce

Add `reply.raw.setHeader(etag, 'dummyetag');` to one path's reply and check if there is a drop in QPS

### Expected Behavior

The QPS should not decrease for paths that already have etag headers

Reproduce the QPS comparison with Fastify 4.15.0, the etag plugin 4.2.0, and Node.js 16.5.1, including a path using reply.raw.setHeader(etag, 'dummyetag'). Compare this with the manual crypto.createHash('sha256') approach and determine whether responses that already have etag headers still incur plugin work; done means the expected QPS is preserved for those paths.

Written by the indexing model from the issue text.