This PR introduces the lookup runtime fields which are used to retrieve
data from the related indices. The below search request enriches its
search hits with the location of each IP address from the `ip_location`
index.
```
POST logs/_search
{
"runtime_mappings": {
"location": {
"type": "lookup",
"lookup_index": "ip_location",
"query_type": "term",
"query_input_field": "ip",
"query_target_field": "_id",
"fetch_fields": [
"country",
"city"
]
}
},
"fields": [
"timestamp",
"message",
"location"
]
}
```
Response:
```
{
"hits": {
"hits": [
{
"_index": "logs",
"_id": "1",
"fields": {
"location": [
{
"city": [ "Montreal" ],
"country": [ "Canada" ]
}
],
"message": [ "the first message" ]
}
}
]
}
}
```
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