Add cache tracing and Redis tracing

This change adds Distributed Tracing support for two new types of events

1. Redis Calls
1. ActiveSupport (Rails) Caching Operations

The intention is to help application developers and infrastructure
SREs to understand the pressure that caching operations can have on
the application when running at scale.

The Redis and Caching spans can be viewed in the Jaeger UI by clicking
the "Trace" link in the performance bar when running on GDK.