- `default` `(key-value-map: {})` - A map of string header names to an array of
  string values. The default headers are set on all endpoints regardless of
  the status code value. For an example, refer to the
  [Listeners custom http response headers](#listener-s-custom-response-headers)
  section.

- `<collective status code>` `(key-value-map: {})` - A map of string header names
  to an array of string values. These headers are set only when the response status
  code falls under the collective status code.
  For example, `"2xx" = {"Header-A": ["Value1", "Value2"]}`, `"Header-A"`
  is set when the http response status code is `"200"`, `"204"`, etc. This overrides
  headers defined at the `default` level.

- `<specific status code>` `(key-value-map: {})` - A map of string header names
  to an array of string values. These headers are set only when the specific status
  code is returned. For example, `"200" = {"Header-A": ["Value1", "Value2"]}`, `"Header-A"`
  is set when the http response status code is `"200"`. This overrides headers defined at
  the `default` and `collective status code` level.
