Add 3k chart example for the hybrid reference architecture

What does this MR do?

The MR adds a new 3k hybrid environment based on the performance testing in gitlab-org/quality/reference-architectures#21 (comment 626349668).

In comparison with 5k, 3k has less Webservice nodes so that it has a similar amount of Puma workers as 3k Omnibus env. The rest is quite similar to 5k.

Service	Nodes	GCP
Webservice	2	n1-highcpu-16
Sidekiq	3	n1-standard-4
Supporting services such as NGINX, Prometheus	2	n1-standard-2

Rails:

• Omnibus 3k: 3 Rails nodes * 5 Puma workers = 15
• Hybrid 3k: 2 webservice nodes * 2 pods each * 4 workers = 16

Sidekiq:

• Omnibus 3k: 4 Sidekiq nodes n1-standard-2
• Hybrid 3k: 3 Sidekiq nodes n1-standard-4

Hybrid 3k:

• 4 total Webservice pods
• 8 total Sidekiq pods

Considerations:

• Rails/Webservice - we need 4 webservice pods. If we go with n1-highcpu-8 to be absolutely aligned with 3k omnibus, then we will need 4 of these nodes with 1 pod each due to webservice memory limit of 5.25G. So went with 2 n1-highcpu-16 which can fit 2 pods each.
• Sidekiq - both 3k and 5k omnibus setups have 4 Sidekiq nodes with n1-standard-2, so left Sidekiq node pool the same as 5k hybrid.

More detailed calculations are in gitlab-org/quality/reference-architectures#21 (comment 624291967)

Related issues

• gitlab-org/quality/reference-architectures#21 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened