message: add CounterLabel prop docs
diff: # CounterLabel
  +## Default example
  ```.jsx
  <CounterLabel>12</CounterLabel>
  <CounterLabel scheme={'gray'}>13</CounterLabel>
  <CounterLabel scheme={'gray-light'}>13</CounterLabel>
  ```
  +## System props
  +
  +CounterLabel components get `COMMON` system props. Read our [System Props](/system-props) doc page for a full list of available props.
  +
  +## Component props
  +
  +| Prop name | Type | Description |
  +| :- | :- | :- |
  +| scheme | String | Pass in 'gray' for a gray background and white text, or 'gray-light' for a lighter gray background and dark text. Omitting the scheme prop renders the default counter scheme |
  +
  export const meta = {displayName: 'CounterLabel'}
  