Migrate project imports total metric to instrumentation class

What does this MR do and why?

Related to #351486 (closed). Follow-up to !84521 (merged).

This MR migrates usage_activity_by_stage.manage.project_imports.total and usage_activity_by_stage_monthly.manage.project_imports.total metrics to an instrumentation class and optimizes the SQL query to have a where clause over multiple import_types.

The class doesn't follow the usual conventions and overrides the behavior of class methods like .relation and .operation in #value and manually constructs a corresponding SQL query, because add operation is not implemented yet (to be in #332713 (closed)) and therefore adding multiple counts is not supported yet.

This is a stop-gap measure to unblock automated Service Ping efforts, which depend on this change of the metric SQL query (see https://gitlab.com/gitlab-data/analytics/-/issues/10163).

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