Add Secure metrics definitions

What does this MR do?

Adds metric definitions for the following metrics:

usage_activity_by_stage_monthly.secure.user_dast_scans
usage_activity_by_stage_monthly.secure.user_sast_scans
usage_activity_by_stage_monthly.secure.user_dependency_scanning_scans
usage_activity_by_stage_monthly.secure.user_container_scanning_scans
usage_activity_by_stage_monthly.secure.user_secret_detection_scans
usage_activity_by_stage_monthly.secure.user_coverage_fuzzing_scans
usage_activity_by_stage_monthly.secure.user_api_fuzzing_scans
usage_activity_by_stage.secure.user_dast_scans
usage_activity_by_stage.secure.user_sast_scans
usage_activity_by_stage.secure.user_dependency_scanning_scans
usage_activity_by_stage.secure.user_container_scanning_scans
usage_activity_by_stage.secure.user_secret_detection_scans
usage_activity_by_stage.secure.user_coverage_fuzzing_scans
usage_activity_by_stage.secure.user_api_fuzzing_scans

Issue

This was requested in a comment on the MR for issue #332557 (closed).

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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