[prometheus] event.dataset is not correct if `data_stream.dataset` is set to not a default value

1. do we actually need to have `event.dataset` hardcoded?

`event.dataset`  was explicitly set for prometheus - https://github.com/elastic/integrations/pull/1249 
need to check original discussion

2. `data_stream.dataset` setting does not affect the mapping for `event.dataset`, in the generated template was still set to `prometheus.collector`