Hi there.
Im newbie in ELK and I ve got a trouble 
I have fresh install of ELK 7.6.2 and I want to log some value from my app via REST. So I make such calls:
POST http://es-host/app-metrics/reqs-count-doc 
{"dt":"2020-05-05T23:43:06.705Z","reqs_count": 41}
everything looks fine - I see all posted data via
    GET http://es-host/app-metrics/_search
    {
        ...
        "hits": {
            ... 
            "hits": [
                {
                    "_index": "app-metrics",
                    "_type": "reqs-count-doc",
                    "_id": "HpcF5nEBNHYsMieKCHIv",
                    "_score": 1,
                    "_source": {
                        "@timestamp": "2020-05-06T00:07:01.551Z",
                        "reqs_count": 90
                    }
                },
                {
                    "_index": "app-metrics",
                    "_type": "reqs-count-doc",
                    "_id": "AZcF5nEBNHYsMieK83Ss",
                    "_score": 1,
                    "_source": {
                        "@timestamp": "2020-05-06T00:08:01.841Z",
                        "reqs_count": 67
                    }
                },  ...
also I see auto-created index in elasticsearch with such mapping:
     {
      "mapping": {
        "reqs-count-doc": {
          "properties": {
            "@timestamp": {
              "type": "date"
            },
            "reqs_count": {
              "type": "long"
            }
          }
        }
      }
    }
Next I created kibana index pattern via WebUI:
- opened Management/Kibana/Index Patterns/Create
 - entered pattern "app-metrics" - got a tip about one matching index
 - chosen @timestamp field as Time Filter Field Name
In created pattern I see declaration of both data fields passed to elastic: @timestamp:date and reqs_count:number. Both are searchable, aggregatable and not excluded. 
But when I try to view this pattern in Discover tab of Kibana - I get nothing (even choosing time period from 100 years ago) and there is an empty list of Available Fields on the left side.
Also when I try to create Visualisation for this data - same empty result is returned...
All actions were made with "elastic" default user.
I have no idea - what was done wrong... appreciate any help.
Thanks in advance.