amtool: Fix behavior of adding silence with duration option

[nekketsuuu]

Closes #2490, also closes #2740.

amtool treats --duration option differently between amtool silence add and amtool silence update; amtool silence add calculates endsAt as time.Now() + duration, while amtool silence update calculates endsAt as startsAt + duration.

According to #1298 (comment), it seems that startsAt + duration is preferable. That also resolves the confusion such as #2490 and #2740.

So I changed behavior of --duration option of amtool silence add. Note that this is a breaking change.

Local test:

$ # This raises an error "silence cannot start after it ends" using amtool 0.23.0.
$ ./amtool silence add alertname=testing --start=2022-01-01T00:00:00Z --duration=30m --comment=testing --alertmanager.url=http://localhost:9093
aa6c19aa-3a14-4760-9075-d726c8bd9cb9
$ ./amtool silence --alertmanager.url=http://localhost:9093
ID                                    Matchers             Ends At                  Created By  Comment  
aa6c19aa-3a14-4760-9075-d726c8bd9cb9  alertname="testing"  2022-01-01 00:30:00 UTC  nek         testing  

It may be better to add tests for these CLI options. Adding tests, however, will be a little large change to cli package because almost all functions of cli package are not exported. So I skipped adding tests in this patch.

Signed-off-by: nekketsuuu <nekketsuuu@users.noreply.github.com>

[jan--f]

Thanks, I think all this makes sense and the changes look good.

Two questions:
Should we add the startsAt default in the help text?
Should we mention this in the release notes? This does change user visible behavior after all.

cc @simonpasquier @roidelapluie

[roidelapluie]

Yes but the release notes are only crafted during the releases, not in individual PR's.

[roidelapluie]

Thank you!

[roidelapluie]

Note that tests are welcome! Functions do not need to be exported to be tested.

[roidelapluie]

Two questions: Should we add the startsAt default in the help text?

I don't think it is really needed.