[Maps] support styles on aggregation fields with _of_ in name

[nreese]

PR #50044 refactor fields and introduced Fields classes. The PR added createField method to AbstractESAggSource that would split the raw field name and aggregation type from the style field name. The problem is that the logic for splitting did not work when the field contained the string _of_. The implementation of createField would have also caused problems if ESTermSource instances ever tried to call the method because the split function is tightly coupled to formatMetricKey implementation.

To view the problem, using the web logs sample data set

1. create a grid source
2. add a metric aggregation on hour_of_day field
3. set fill color to use the above aggregation field. Notice that all grids have empty color.

This PR fixes the problem by removing the need to split the raw field and aggregation type from the field name. This is done by just using the getMetricFieldForName function which returns fields from the existing metrics list which stores both the raw field name and aggregation type.

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: fb3ed56

[thomasneirynck]

Thanks for simplification, and obviously the fix of a long-standing bug flying under the radar.

See below for comment.

[thomasneirynck]

Great simplification! The main consequence now is that EsAggSource#createField is no longer a factory-function and creates inconsistencies with the other implementations of VectorSource#createField.

1. By using the getMetricFieldForName, which only will return previously created objects, this continues to work as expected because all object-instances are guaranteed to be read-only, so reuse is not an issue. But that's implicit and never enforced (ie. redux requires persistence to always flow through the store, and we enforce this through code-review primarily).

2. ESAggSource#createField can only construct fields that are actually available in the EsAggSource, which is inconsistent with all the other implementations. It is a fine limitation, but implementations should be consistent. e.g. consider EMS-sources disallowing creating fields which are not present in the manifest, ES-source disallowing creating fields which are not present in the index-pattern, etc...).

This was the reason we weren't reusing getMetricFieldForName here originally, instead doing a manual lookup for a custom label first.

3. createField can no longer be used transparently, regardless of caller. e.g. sometimes it is used in the Source#constructor as a shorthand (which would require creation-of-fields from scratch). Other times, it's used from outside (like here, where the bug arises

   kibana/x-pack/legacy/plugins/maps/public/layers/styles/vector/vector_style.js

   Lines 455 to 457 in 05f765a

   	return this._source.createField({
   	fieldName: fieldDescriptor.name
   	});

   ).

Maybe consider (?)

• Introduce something like VectorSource#getFieldByName. EsAggSource can implement it just with getMetricFieldByName, the other sources can just delegate to use createField by default.
• Modify

  kibana/x-pack/legacy/plugins/maps/public/layers/styles/vector/vector_style.js

  Lines 455 to 457 in 05f765a

  	return this._source.createField({
  	fieldName: fieldDescriptor.name
  	});

  to return this._source.getFieldByName(descriptor.name)

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 638b840

[thomasneirynck]

Thx @nreese, as discussed offline, I'll follow up with this

[nreese]

closing in favor of #54965