Loki: Add route_randomly to Redis options

[wtchangdm]

What this PR does / why we need it:

Current Redis client only reads from master node, which makes master node under heavy pressure.

Usually, one or more read replicas are expected on production environments.

This PR helps to increase the utilization of read replicas.

Which issue(s) this PR fixes:
Fixes #8811

Special notes for your reviewer:

• The route_randomly option defaults to false, which matches the same behavior in previous versions, hence no breaking change.
• User needs to explicitly set route_randomly to true in order to use it.
• The purpose of this PR is to evenly distribute the read requests, so only RouteRandomly is used and not the following:
  • ReadOnly (which only reads from read replicas)
  • RouteByLatency (requests probably won't be even since not all Loki components are deployed evenly separate)

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[dannykopping]

LGTM, please just add some more documentation
Thanks for the contribution @wtchangdm!

[dannykopping]

Thanks! I touched up the wording a little.
Please merge main into your branch and rerun make doc and commit the result.

[wtchangdm]

Thanks @dannykopping, I've regenerated the doc and rebased on main.

[dannykopping]

LGTM

[wtchangdm]

Thanks again @dannykopping!