Add instrumentor for Redis

Add instrumentor for Redis. Below are existing instrumentor implementations:

[DataDog](https://github.com/open-telemetry/opentelemetry-auto-instr-python/tree/master/ddtrace/contrib/redis)
[SignalFx](https://github.com/signalfx/signalfx-python-tracing/tree/master/signalfx_tracing/libraries/redis_)