This commit adds a new pipeline aggregation that allows correlation within the aggregation frame work in bucketed values.
The initial function is a `count_correlation` function. The purpose of which is to correlate the count in a consistent number of buckets with a pre calculated indicator. The indicator and the aggregated buckets should related to the same metrics with in documents.
Example for correlating terms within a `service.version.keyword` with latency percentiles. The percentiles and provided correlation indicator both refer to the same source data where the indicator was previously calculated.:
```
GET apm-7.12.0-transaction-generated/_search
{
"size": 0,
"aggs": {
"field_terms": {
"terms": {
"field": "service.version.keyword",
"size": 20
},
"aggs": {
"latency_range": {
"range": {
"field": "transaction.duration.us",
"ranges": [<snip>],
"keyed": true
}
},
"correlation": {
"bucket_correlation": {
"buckets_path": "latency_range>_count",
"count_correlation": {
"indicator": {
"expectations": [<snip>],
"doc_count": 20000
}
}
}
}
}
}
}
}
```
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