add activity metric to prometheus-labels.yaml

[muuki88]

A new metric was added, but the prometheus labels were missing

🔧 Type of changes

• new bid adapter
• bid adapter update
• new feature
• new analytics adapter
• new module
• module update
• bugfix
• documentation
• configuration
• dependency update
• tech debt (test coverage, refactorings, etc.)

✨ What's the context?

A new metric was added ( activity ) without adding a prometheus match rule

🧠 Rationale behind the change

Prometheus labels work as expected

🧪 Test plan

We run it in production already

🏎 Quality check

• Are your changes following our code style guidelines?
• Are there any breaking changes in your code? - there are, but it's broken before, so meh.
• Does your test coverage exceed 90%?
• Are there any erroneous console logs, debuggers or leftover code in your changes?

[Net-burst]

LGTM