* Fix add_labels flattening of arrays values

The processor was not working as described in the docs. For example:

```yaml
processors:
  - add_labels:
      labels:
        number: 1
        with.dots: test
        nested:
          with.dots: nested
        array:
          - do
          - re
          - with.field: mi
```

```json
{
  "labels": {
    "array": [
      "do",
      "re",
      {
        "with": {
          "field": "mi"
        }
      }
    ],
    "nested.with.dots": "nested",
    "number": 1,
    "with.dots": "test"
  }
```

```json
{
  "labels": {
    "number": 1,
    "with.dots": "test",
    "nested.with.dots": "nested",
    "array.0": "do",
    "array.1": "re",
    "array.2.with.field": "mi"
  }
}
```

* Expose ucfg FlattenedKeys() in common.Config

(cherry picked from commit 1dd9714)