NebulexRedisAdapter does not implement `c:stats/0`

[Simon Cocking]

Given:

defmodule MyCache do
  use Nebulex.Cache,
    otp_app: :nebulex,
    adapter: NebulexRedisAdapter
end

then

iex> MyCache.stats()
** (UndefinedFunctionError) function MyCache.stats/0 is undefined or private, but the behaviour Nebulex.Cache expects it to be present
    (my_app 0.1.0) MyCache.stats()

This leaves us unable to obtain Telemetry metrics for this cache.

[Carlos Andres Bolaños R.A.]

Yeah, the adapter does not implement the Nebulex.Adapter.Stats behaviour, this will be supported soon. Now that it emits the Telemetry span events, it is easy to implement stats similar to the Nebulex.Adapters.Local by using the provided Telemetry handler Nebulex.Telemetry.StatsHandler.