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)

[maartenberg]

I agree with Rick's suggestion for the metrics design: I think it's better to alter the existing hoff_project_merged_pull_requests to add a priority label, rather than introducing a new metric to track specifically the PRs merged with priority.

I also agree with Rick that only tracking attempted and completed merges can be used to detect failures: if we want to track failures we should probably add a new metric to track merge failures, grouped by repository, priority and failure reason (the options in

hoff/src/Git.hs

Lines 200 to 206 in adb1b34

	data GitIntegrationFailure
	= MergeFailed
	| RebaseFailed
	| WrongFixups
	| EmptyRebase
	| FailedForcePush Text
	deriving (Show, Eq, Generic)

).

We could also consider keeping a gauge with the number of PRs in the merge queue for every repository, but this is probably more cumbersome to implement.