Draft: Slack/Mattermost integration activity counter

What does this MR do?

Adds a usage data counter to count activity of the Slack and Mattermost integration.

The related issue is only about adding it for Slack bot both integrations are deeply connected, so we can add it to both integrations in one go.

Related issues

• #297402 (closed)

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