Add 'flatten' parameter to object mappers

[romseygeek]

Objects with flatten:true will automatically flatten all sub-objects into dot-delimited
fields, allowing users to specify field names that are dot-prefixes of other fields.

For example, say you have a set of fields metric.value, metric.value.min and
metric.value.max. Setting flatten:true on the top-level metric object will
allow value, value.min and value.max to all appear as leaf fields:

"properties" : {
  "metric" : {
    "type" : "object",
    "flatten" : true,
    "properties" : {
      "value" : { "type" : "long" },
      "value.max" : { "type" : "long" },
      "value.min" : { "type" : "long" },
    }
  }
}

Objects with flatten:true cannot contain further object mappers.

Relates to #63530

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[romseygeek]

I want to let CI chew on this and I'm pretty sure there are further cleanups I can make in DocumentParser (which now has a bunch of unused code) and related tests, but it would be good to get some feedback on whether we think this is the right way of implementing this, at least as an external API.

Note that as of now I have explicitly not implemented this on nested mappers, but it would be easy enough to extend it there.

[romseygeek]

The vast bulk of this logic is now moved into the new ObjectMapper.Builder#addDynamic() method. Instead of creating concrete object mappers and recursively merging them, instead we pass each mapper to a root object builder. The builder checks to see if there are any dots in the mapper's field name, and if there are and the object is not flattened then we strip off the first object name, create the relevant object builder if it doesn't already exist, and pass the shortened name along with the mapper down to the next level. If the object is flattened, then we just add the mapper directly underneath it.

[romseygeek]

This part allows us to handle deeply nested object paths that become flattened at some arbitrary point. getDynamicParentMapper will stop building intermediate object mappers if one of them is flattened, and this logic builds up the final field name from the point of the flattened object.

[romseygeek]

Not strictly related, but it made debugging this so much easier that I thought it worth leaving in.

[romseygeek]

We don't need to set all the dynamicTemplates, etc, because we're creating a Builder which already has all these set to implicit defaults.

[elasticsearchmachine]

Hi @romseygeek, I've created a changelog YAML for you.

[javanna]

Superseded by #86166 , made possible by #79922 and #81449.