Add option to exlcude alerts from the results

[martialblog]

This adds an option to exclude alerts from the results.

The use case behind this: sometimes you want to define
a so called Watchdog or DeadMansSwitch alert that is always
firing, in order to monitoring that the alerting is working.

When such a Watchdog is defined the list of all alerts will
always be Critical. Thus we add a flag to exclude certain alerts.

See #61

I want to tackle the issue in two steps, this is the first. The second is a feature to specify the expected exit code.

[martialblog]

How often can a person misspell "exclude"! 🤦

[martialblog]

@RincewindsHat The commit is ontop the others since otherwise I would have to cleanup the merge conflict and I'm so so so so lazy. Just ignore the others

[wattebausch]

Looks very good. Even works with multiple excludes

Check with "alert -P (--problems)"

./check_prometheus -H metrics.some-probelm-cluster.example.com -p 80 alert -P 
[CRITICAL] - 1 Alerts: 1 Firing - 0 Pending - 0 Inactive
\_ [CRITICAL] [Watchdog] is firing - value: 1.00

 ./check_prometheus -H metrics.some-probelm-cluster.example.com -p 80 alert -P --exclude-alert Watchdog
[UNKNOWN] - 0 Alerts: 0 Firing - 0 Pending - 0 Inactive

Only "alert"

./check_prometheus -H metrics.some-probelm-cluster.example.com -p 80 alert 
[CRITICAL] - 126 Alerts: 1 Firing - 0 Pending - 125 Inactive
\_ [OK] [FluxHelmReleaseNotReady] is inactive
..
\_ [OK] [TargetDown] is inactive
\_ [CRITICAL] [Watchdog] is firing - value: 1.00
\_ [OK] [InfoInhibitor] is inactive


./check_prometheus -H metrics.slfl28652.wsl.ch -p 80 alert --exclude-alert Watchdog
[OK] - 125 Alerts: 0 Firing - 0 Pending - 125 Inactive
\_ [OK] [FluxHelmReleaseNotReady] is inactive
..
\_ [OK] [TargetDown] is inactive
\_ [OK] [InfoInhibitor] is inactive
..

no impact if Cluster is allready okay. verry nice

 ./check_prometheus -H metrics.clean-cluster.example.com -p 80 alert -P --exclude-alert Watchdog
[UNKNOWN] - 0 Alerts: 0 Firing - 0 Pending - 0 Inactive

/check_prometheus -H metrics.clean-cluster.example.com -p 80 alert --exclude-alert Watchdog
[OK] - 125 Alerts: 0 Firing - 0 Pending - 125 Inactive
\_ [OK] [FluxHelmReleaseNotReady] is inactive
..
\_ [OK] [TargetDown] is inactive
\_ [OK] [InfoInhibitor] is inactive

[RincewindsHat]

This is not ideal. If the regex is invalid (as hard as it might be to achieve that), this line panics.
I think I would prefere a controled exit with a helpful message here.
the ..MustCompile functions are preferable for static expressions.

[martialblog]

Yes, you're right. I updated the implementation

[RincewindsHat]

Looks good :-)