promtool: Buffer piped unit test rule input to a tempfile

[dgl]

Fixes #13785.

[machine424]

I haven't checked the PR yet, but hopefully it'll close https://github.com/prometheus/prometheus/issues/6222 as well.

[dgl]

@machine424 this one is specific to test rules. It's not quite the same as that issue, as it's allowing referencing "/dev/stdin" in the test data itself, rather than whether no file argument implies an implicit stdin.

[machine424]

I read test rules instead of check rules, my bad.

[dgl]

Thanks, updated.

Should we mention this in the docs

Yes, I've added a bit to the docs.

[roidelapluie]

I think this is not a good fix. The fix should be to read the file once and not making exceptions for pipes etc .

[bboreham]

Hello from the bug-scrub: @dgl do you have any further thoughts?