Ensure accessModifier is in all example config files (#456)

### Motivation

We want all examples to explicitly spell out the `accessModifier` in the
`openapi-generator-config.yaml`. There was just one missing:

```console
% git ls-files "**/openapi-generator-config.yaml" | xargs grep -L accessModifier
Examples/MetricsMiddleware/Sources/HelloWorldVaporServer/openapi-generator-config.yaml
```

### Modifications

Add missing `accessModifier` to metrics middleware example.

### Result

All examples have `accessModifier` in their
`openapi-generator-config.yaml`.

### Test Plan

None.

Author: simonjbeaumont

Examples/MetricsMiddleware/Sources/HelloWorldVaporServer/openapi-generator-config.yaml

@@ -1,3 +1,4 @@
 generate:
   - types
   - server
+accessModifier: internal