Add hostname to the @metadata

Currently, some inputs add the `hostname` of the indexer, which means that we can use this value in the filters to add them as tags, or do any other operations.

Since the hostname is being added at the input level, each input (plugin) need to add the same code in order to make this happen.

My proposal is to **add the host to the @metadada** field at every time at the input core level rather than the plugin level.

As discussed with @ph today, we will continue our discussion in this thread.