Redis observability on GKE - prometheus exporters

This is a prerequisite for #1611 (closed).

redis_exporter

In order to get solid metrics for redis we will need to get redis_exporter running inside the helm chart.

The bitnami/redis helm chart supports a metrics.enabled setting which adds another container using the bitnami/redis-exporter image.

process-exporter

We may also want to get process-exporter in place, in order to get per-thread metrics which we use for the saturation SLI.

The helm chart supports a replica.sidecars setting which allows adding arbitrary containers to the pod. We can use the upstream ncabatoff/process-exporter image for this purpose.