Add issue importers usage pings

What does this MR do?

Depends on !40130 (merged)

This MR adds usage ping data for Project Imports of the following importers:

• Jira
• Fogbugz
• Phabricator

Mentions #235425 (closed)

Query Explain

Jira

With timerange

explain SELECT COUNT(DISTINCT "jira_imports"."user_id") FROM "jira_imports" WHERE "jira_imports"."created_at" BETWEEN '2020-07-29 10:31:57.337227' AND '2020-08-26 10:31:57.337423' AND "jira_imports"."user_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/ZJGA

Without timarange

explain SELECT COUNT(DISTINCT "jira_imports"."user_id") FROM "jira_imports" WHERE "jira_imports"."user_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/b0JB

Fogbugz

With timerange

explain SELECT COUNT(DISTINCT "projects"."creator_id") FROM "projects" WHERE "projects"."import_type" = 'fogbugz' AND "projects"."created_at" BETWEEN '2020-07-27 09:18:09.605426' AND '2020-08-24 09:18:09.605489' AND "projects"."creator_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/Uskb

Without timerange

explain SELECT COUNT(DISTINCT "projects"."creator_id") FROM "projects" WHERE "projects"."import_type" = 'fogbugz' AND "projects"."creator_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/wOTN

Phabricator

With timerange

explain SELECT COUNT(DISTINCT "projects"."creator_id") FROM "projects" WHERE "projects"."import_type" = 'phabricator' AND "projects"."created_at" BETWEEN '2020-07-27 09:18:09.605426' AND '2020-08-24 09:18:09.605489' AND "projects"."creator_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/qo1j

Without timerange

explain SELECT COUNT(DISTINCT "projects"."creator_id") FROM "projects" WHERE "projects"."import_type" = 'phabricator' AND "projects"."creator_id" BETWEEN 1 AND 10000

https://explain.depesz.com/s/fz0n

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team