k6: configurable amount of time for each replica group, to achieve co…

[NickAnge]

What this PR does

In this PR we add the ITERATIONS_PER_HA_REPLICAS_GROUP at the [load-testing-with-k6.js](https://github.com/grafana/mimir/compare/nickange/ha_tracker/k6/configurable-time-ha-replica-grouping?expand=1#diff-1ab093505a4e13c366d835242521a94aeeef8fd2ec31863b40a8cf6d30cf9276), in order to achieve constant changes at the key entries of the HA_Tracker.

Command being used

./k6 run load-testing-with-k6.js -e K6_WRITE_HOSTNAME="localhost:8000" -e K6_READ_HOSTNAME="localhost:8007" -e K6_SCRAPE_INTERVAL_SECONDS=15 -e K6_HA_REPLICAS=200 -e K6_WRITE_SERIES_PER_REQUEST=1 -e K6_ITERATIONS_PER_HA_REPLICAS_GROUP=4  -e K6_WRITE_TENANT_ID="mimir-memberlist-test" -v

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.