Fix #34: use fields.yml for export template so properties and @timest…

[deepujain]

Summary of fix
Fixes #34. The export template output previously had empty "properties": {}, which led to @timestamp being mapped as a string 1. This change wires the beat to the repo’s fields definition so the exported template has the right mappings.

Changes made

• \meta/fields.yml – Define fields used for template generation: @timestamp (type date), beat (group: hostname, name, version as keyword), type (keyword), and existing counter (long).
• nvidiagpubeat.yml – Set setup.template.fields: "_meta/fields.yml" so export template and setup use this file instead of the empty embedded asset.
• README.md – Add an “Export index template” section: run ./nvidiagpubeat -c nvidiagpubeat.yml export template and note that the config points at _meta/fields.yml for correct mappings.

Verification
Run from repo root:
`
./nvidiagpubeat -c nvidiagpubeat.yml export template
{
"index_patterns": [
"nvidiagpubeat-6.5.5-*"
],
"mappings": {
"doc": {
"_meta": {
"version": "6.5.5"
},
"date_detection": false,
"dynamic_templates": [
{
"strings_as_keyword": {
"mapping": {
"ignore_above": 1024,
"type": "keyword"
},
"match_mapping_type": "string"
}
}
],
"properties": {
"@timestamp": {
"type": "date"
},
"beat": {
"properties": {
"hostname": {
"ignore_above": 1024,
"type": "keyword"
},
"name": {
"ignore_above": 1024,
"type": "keyword"
},
"version": {
"ignore_above": 1024,
"type": "keyword"
}
}
},
"counter": {
"type": "long"
},
"type": {
"ignore_above": 1024,
"type": "keyword"
}
}
}
},
"order": 1,
"settings": {
"index": {
"mapping": {
"total_fields": {
"limit": 10000
}
},
"number_of_routing_shards": 30,
"refresh_interval": "5s"
}
}
}

`

The JSON now has non-empty mappings.doc.properties with @timestamp as date, plus beat, counter, type.

Request to eBay maintainer
Please review and merge this PR when convenient. Thank you.