Inhibit not work when two related alert first arrived at same time

[Allenzh li]

Hi,

I have use cortex which use prometheus and alertmanager as infrastructure in my environment, when i test inhibit find a some time it is not work as expect.

In a case, I run cortex and set two rules:

1. go_threads > 0   severity 4

2. go_threads > 10 severity 1

My inhibit rules:

- source_match: 

      alert_severity: "1" 

 target_match: 

      alert_severity: "4" 

 equal: 

      .....

the go_threads value is 17

I find alertmanager receive two alert at same time and inhibit rules not work