[Maps] term join metrics with same aggregation and field have name collision

[nreese]

https://discuss.elastic.co/t/multiple-term-join-in-elastic-maps/227550

Create a ES Documents source from kibana_sample_data_logs data set. Set scaling to top terms and split on machine.os. In the screen shot below, there are 2 configured terms join that use a where clause to create different results. One is looking for the avg bytes when bytes is greater than 10000 and the other is looking for avg bytes when bytes is less then 10000. Notice how both avg bytes metrics are the same. They should have different values.

This is caused because the metrics from the separate terms joins are not name spaced. So both store their values in the same feature property key __kbnjoin__avg_of_bytes_groupby_kibana_sample_data_logs.machine.os.keyword. Metric key is created here - https://github.com/elastic/kibana/blob/7.7/x-pack/legacy/plugins/maps/public/layers/sources/es_term_source.js#L66

Saved object joins configuration

"joins": [
      {
        "leftField": "machine.os",
        "right": {
          "id": "9055b4aa-136a-4b6d-90ab-9f94ccfe5eb5",
          "indexPatternTitle": "kibana_sample_data_logs",
          "term": "machine.os.keyword",
          "metrics": [
            {
              "type": "avg",
              "field": "bytes"
            }
          ],
          "whereQuery": {
            "query": "bytes > 10000",
            "language": "kuery"
          },
          "indexPatternRefName": "layer_1_join_0_index_pattern"
        }
      },
      {
        "leftField": "machine.os",
        "right": {
          "id": "9a7f4e71-9500-4512-82f1-b7eaee3d87ff",
          "indexPatternTitle": "kibana_sample_data_logs",
          "term": "machine.os.keyword",
          "whereQuery": {
            "query": "bytes < 10000",
            "language": "kuery"
          },
          "metrics": [
            {
              "type": "avg",
              "field": "bytes"
            }
          ],
          "indexPatternRefName": "layer_1_join_1_index_pattern"
        }
      }
    ]

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

The tricky part to fixing this will be preserving existing saved objects. Styling links to the join field by the aggKey so if the aggKey changes, then we will have to migrate any styling properties using the agg key.

For example, below is the fill color styling descriptor for the saved object configuration. __kbnjoin__avg_of_bytes_groupby_kibana_sample_data_logs.machine.os.keyword would need to be updated if the aggKey includes the join id.

"style": {
      "properties": {
        "fillColor": {
          "type": "DYNAMIC",
          "options": {
            "color": "Blues",
            "colorCategory": "palette_0",
            "field": {
              "name": "__kbnjoin__avg_of_bytes_groupby_kibana_sample_data_logs.machine.os.keyword",
              "origin": "join"
            },
            "fieldMetaOptions": {
              "isEnabled": true,
              "sigma": 3
            },
            "type": "ORDINAL"
          }
        },