Missing endpoint support for cluster operations

[zopyx]

The Python client has no support for cluster operations like metrics, health and stats although they are documented :)

[DominikSerafin]

Until this gets better support, this can be achieved by calling API directly, e.g.:

• client.api_call.get('/collections')
• client.api_call.get('/metrics.json')

[tharropoulos]

This issue hasn't seen traction in a while, but this has been added in the latest version by #77 for metrics and the rest of the API has been supported by v.1.0.0.