This is enabled by adding a new dynamic_templates_params option to the bulk request header. It can be used in combination with the dynamic_templates option to provide parameters that can be used within a dynamic template.

The dynamic template can control which parameters it accepts and can provide a default value in case clients don't provide the parameters in the bulk request. If no default value is provided, and the bulk request also doesn't specify the parameter, the value is kept as it is defined in the template (including braces).

For example, the dynamic template can define a unit placeholder.

{
  "dynamic_templates": [
    {
      "gauge_long": {
        "mapping": {
          "type": "long",
          "time_series_metric": "gauge",
          "meta": {
            "unit": "{{unit}}"
          }
        }
      }
    }
  ]
}

Clients can then set provide a value for the unit in the bulk request header like so:

{"create": {"dynamic_templates":{"metrics.system.memory.usage":"gauge_double"},"dynamic_template_params":{"metrics.system.memory.usage":{"unit": "By"}}}}
{"metrics": {"system.memory.usage": 42}}

Placeholders for which no parameters are specified will evaluate to an empty string.

I chose a {{placeholder}} syntax over {placeholder} to avoid ambiguity as {percent} is actually a literal value that we expect to use which is not meant to be interpreted as a placeholder. This is also consistent with the syntax we use in other places like ingest pipelines (mustache and reroute processor placeholders). However, at the moment, dynamic templates support some parameters with only a single pair of braces: {name} and {dynamic_type}. I've added support for specifying these in double braces as well to maintain consistency (while still supporting the single brace version). Interestingly, we've documented that we already support this syntax: https://www.elastic.co/docs/manage-data/data-store/mapping/dynamic-templates

Use the {{name}} and {{dynamic_type}} template variables in the mapping specification as placeholders.

However, it's not actually true (yet).