Show rule evaluation errors on rules page

[matthiasr]

Proposal

On the /rules UI page, show the last evaluation status and any errors for each rule. This is similar to how the status and errors for targets are shown on /targets.

Use case. Why is this important?

Humans are not perfect, and sometimes rule evaluation fails. This cannot be fully solved by testing beforehand, since e.g. many-to-many matching errors and timeouts are data dependent.

Right now, we need to find and look at the logs to see this error. Surfacing the last error would make this much easier. It's not a great help for fleetingly broken rules but most of the time when they break they stay broken for a while.

• Logs:

2018-08-01_11:42:43.52603 level=warn ts=2018-08-01T11:42:43.525846289Z caller=manager.go:343 component="rule manager" group=<redacted> msg="Evaluating rule failed" rule="record: <redacted> \nexpr: (<redacted> / on(a, b) <redacted>)\n  * 100\n" err="many-to-many matching not allowed: matching labels must be unique on one side"

[matthiasr]

This would also implement one aspect of #1481.

[brian-brazil]

Sounds like a good idea to me.

[noqcks]

/assign moi

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.