WIP: Add promjs

What does this MR do?

Add promjs.

The usecase I presented here as an example is bad but is an easy metric to play with whilst dev/test.

This POSTs every 15s to gon.gatewayEndpoint with a prometheus data.

We will need an aggregation gateway.

Example request data:

# HELP toggle_sidebar Counts right sidebar toggles
# TYPE toggle_sidebar counter
toggle_sidebar 13

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?