Star tree mapping changes

[bharath-techie]

Description

This PR contains the changes for star tree field mapping with feature flag protection. To make the changes extensible to other multi-field/composite/datacube type of indices in future, I've generalized the implementation under 'CompositeIndex'.

Mappings have a new section 'composite' under which multi field mappings such as 'star tree' can be defined. All the fields associated with metrics and dimensions must be present in 'properties' section. [ This is even applicable for update mapping API - but right now , its blocked for star tree - star tree can be only specified during creation of index ]

Min version :

"mappings": {
        "dynamic": "strict",
        "_source": {
            "enabled": true
        },
        "composite": {
            "startree1": {
                "type": "star_tree",
                "config": {
                    "ordered_dimensions": [
                       {
                        "name": "@timestamp"
                       },
                       {
                        "name": "status"
                       }
                    ],
                    "metrics": [
                      {
                        "name": "size"
                       },
                       {
                        "name": "request_rate"
                       }
                    ]
                }
            }
        },
        "properties": {
            "@timestamp": {
                "format": "strict_date_optional_time||epoch_second",
                "type": "date"
            },
            --------
            
        }
    }

}

And the defaults will be filled for the above fields.

NOTE : We will tune the defaults throughout the development of the star tree index.

Defaults : Timestamp field intervals = [ Minute, Hour ] Default Metrics for each metric field = [ SUM, COUNT, AVG, MIN, MAX ]

Complete version :

"mappings": {
        "dynamic": "strict",
        "_source": {
            "enabled": true
        },
        "composite": {
            "startree1": {
                "type": "star_tree",
                "config": {
                    "ordered_dimensions": [
                        {
                            "name": "timestamp",
                            "calendar_intervals": [
                                "day",
                                "month"
                            ]
                        },
                        {
                            "name": "size"
                        }
                    ],
                    "metrics": [
                        {
                            "name": "size",
                            "stats": [
                                "sum",
                                "avg",
                                "min"
                            ]
                        }
                    ]
                }
            }
        },
        "properties": {
            "@timestamp": {
                "format": "strict_date_optional_time||epoch_second",
                "type": "date"
            }
           -----------
        }
    }

Validations

Apart from basic validations based on user input :

- We will start with support for one field mapping under composite index , so technically one star tree index per source index.
- Maximum of 10 dimensions [will fine tune this]
- For date fields - maximum for 3 intervals
- All dimension fields and metric fields must be aggregation compatible [ doc values + field data supported ]
- We will add a limit on number of metrics later on.

Open questions

• Saw sort valiadations on 'shrink index' api etc - where I'll also add similar validations. What other index APIs we need to restrict creation / support of star tree ?

Related Issues

https://github.com/opensearch-project/OpenSearch/issues/13875 https://github.com/opensearch-project/OpenSearch/issues/14386

Check List

• [ ] Functionality includes testing.
• [ ] API changes companion pull request created, if applicable.
• [ ] Public documentation issue/PR created, if applicable.

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