[Fleet] query_string queries fail because index.query.default_field set to message for metric indices

[simianhacker]

Kibana version:
master

Elasticsearch version:
master

Server OS version:
Mac OS X

Describe the bug:

The index setting index.query.default_field is set to message for all the metric indices. This causes issues with query string queries because message is not an available field for metric indices. This means the data is unusable inside Grafana, discovered originally by @jamiesmith.

Steps to reproduce:

1. Setup agent and start sending system data
2. Run the following request in the Dev Console:

GET metrics-*/_search
{
  "query": {
    "query_string": {
      "analyze_wildcard": true, 
      "query": "*"
    }
  }
}

Expected behavior:

The query above should return data.

Screenshots (if relevant):

Any additional context:

Here is the code that needs to change: https://github.com/elastic/kibana/blob/master/x-pack/plugins/fleet/server/services/epm/elasticsearch/template/template.ts#L311

[elasticmachine]

Pinging @elastic/fleet (Feature:Fleet)

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)