Add operation ctx safety for prometheus tracing

Last year a PR on the main repo https://github.com/99designs/gqlgen/pull/2709 fixed a ```panic``` related to the Apollo tracing. The prometheus tracing here in gqlgen-contrib seems to suffer from the exact same issue.

I would guess that this same check:

```golang
if !graphql.HasOperationContext(ctx) {
return next(ctx)
}
```

Should be added before the call to ```GetOperationContext``` [here](https://github.com/99designs/gqlgen-contrib/blob/7be2c32e86b8c154211b86823c35bd56e08ab831/prometheus/prometheus.go#L129).