Hey,
you may want to concatenate the array values of all buckets see https://www.elastic.co/guide/en/elasticsearch/reference/5.1/search-template.html#_concatenating_array_of_values
That would be sth like (on top of my head, not tested)
"text" : "Encountered: {{ctx.payload.hits.total}} in the last hour on {{ctx.payload.aggregations.host.buckets}}{{key}}{{/ctx.payload.aggregations.host.buckets}}
--Alex