Add attempted merge and hotfix metrics

[tijmendj]

Used for finding change failure rates. Can also be used to find if merges failed.

[Riscky]

We never explicitly register failures, just (successful) attempts. Do we also want to register failures?

Also I'd recommend registering the priority as a label, so you'd get:

hoff_project_merge_attempted_pull_requests{priority="high"}
hoff_project_merge_attempted_pull_requests{priority="normal"}

instead of two separate metrics

[tijmendj]

We never explicitly register failures, just (successful) attempts. Do we also want to register failures?

I assumed that your failures can be derived from subtracting the successful attempts from your total attempts. If that's unwise we could look into tracking failures separately instead

[Riscky]

I assumed that your failures can be derived from subtracting the successful attempts from your total attempts. If that's unwise we could look into tracking failures separately instead

Not really, while the attempt it ongoing it will be added to one of the two metrics, but not the other (because we don't know if it will succeed yet)