Check for duplicates in template generation

[urso]

Having multiple dynamic_templates with the same field name in templates can lead to bugs. For example: elastic/apm-server#2693, elastic/apm-server#2693 (comment)

The Elasticsearch team is discussing to disallow multiple dynamic templates: elastic/elasticsearch#53326

Beats template generator should check for duplicates and fail (or at least warn) if we detect duplicates.

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[simitt]

In APM Server we currently have the same key in the dynamic template for e.g. labels:

    "mappings" : {
      "dynamic_templates" : [
        {
          "labels" : {
            "path_match" : "labels.*",
            "mapping" : {
              "type" : "keyword"
            },
            "match_mapping_type" : "string"
          }
        },
        {
          "labels" : {
            "path_match" : "labels.*",
            "mapping" : {
              "type" : "boolean"
            },
            "match_mapping_type" : "boolean"
          }
        },
        {
          "labels" : {
            "path_match" : "labels.*",
            "mapping" : {
              "scaling_factor" : 1000000,
              "type" : "scaled_float"
            },
            "match_mapping_type" : "*"
          }
        },
        ...
     ]
  }

The logic for creating the dynamic template with multiple entries for the same key in libbeat could be adapted to e.g. adding a sequence to the key if the same key exists multiple times.
The corresponding definition in _meta/fields.yml looks like the following

    - name: labels
      type: object
      object_type_params:
        - object_type: keyword
        - object_type: boolean
        - object_type: scaled_float
          scaling_factor: 1000000
      dynamic: true