Add count of created_service_accounts,28d

Related to #517373 (closed)

What does this MR do and why?

• Adds a metric for count.created_service_accounts per 28 days
• Adds a metric for count.active_service_accounts per 28 days

Metric	Action	GitLab.com	Self-Managed	Dedicated
count.created_service_accounts	POST
count.active_service_accounts	POST & state: 'active'

Queries

• count.created_service_accounts

Warm Cache = 8 ms

SELECT COUNT(DISTINCT users.id) 
FROM users
WHERE users.user_type = 13 
AND users.created_at BETWEEN (NOW() - INTERVAL '28 days') AND NOW()

• count.active_service_accounts

Warm Cache = 7.6 ms

SELECT COUNT(DISTINCT users.id)
FROM users
WHERE users.user_type = 13 
AND users.state = 'active' 
AND users.updated_at BETWEEN (NOW() - INTERVAL '28 days') AND NOW()

How to set up and validate locally

1. Change the SQL queries in the .rb files & run the rspecs
2. Confirm they fail correctly

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.