Clarify scope of doc/how/prometheus.md
What does this Merge Request do?
This page covers 2 fairly independent topics but the first sentence makes you think it's only talking about 1 of them and as a consequence the reader may (like me) stop reading and assume it's not the content they're looking for. This also correctly separates them as distinct sections by nesting headers relevant to each section.
Merge Request checklist
- [-] Tests added for new functionality. If not, please raise Issue to follow-up.
- [-] This change is backward compatible. If not, please include steps to communicate to our users.
- [-] Documentation added/updated, if needed.
- [-]
gdk doctor
test added, if needed.