Commit Message:
Un-Reverts 048583b, with fix for high cpu consumption.

Additional Description:
This PR implements fault injection for Redis; specifically delay and error faults (which themselves can have delays added). I chose not to implement a separate filter after discussing with Henry; we concluded that the faults we felt were useful didn't need many levels- just a delay on top of the original fault, if any. In addition, as the Redis protocol doesn't support headers that makes it a bit different again from Envoy's http fault injection. This PR fixes the issue previously seen with redis fault injection where excessive cpu was used on connection creation.

Faults record metrics on the original request- and the delay fault adds extra latency which is included in the command latency for that request. Also, faults can apply only to certain commands.
Future work: Add several other faults, including cache misses and connection failures.

Risk Level: Medium

Testing:

• Unit tests on fault manager and command splitter.
• Testing on local machine. For 3 faults, the faults/requests after a few dozen requests reflected the desired configuration.
• Specific testing for high cpu usage on local machine. Used rpc-perf with various combinations of connections and clients to simulate higher load than normal testing with single requests.

Docs Changes: yes- updated Redis configuration section with notes on metrics and fault injection proper in docs/root/configuration/listeners/network_filters/redis_proxy_filter.rst .

Release Notes: yes, under 1.16 (pending)