feat: Support negative duration in new function ParseDurationAllowNegative

[iamhalje]

This PR extend the ParseDuration function to support negative durations.

negative durations need be useful in Loki alerts templates where time offsets such as '-5m' are common in range calculations and comparisons.

Changes:

• update ParseDuration to handle negative values like -1h, -30m, etc.
• preserves compatibility with existing valid duration strings.
• includes error checks for unit order and duration overflows.

related to promethes/template changes for Loki alerting - see prometheus/prometheus#16669

[beorn7]

Looks good in general. I just have a nit about redundant code comments and overly short variable names.

However, there is also a more general concern: My assumption was this function is just used for templating, but it is used for many other purposes, too. It is not unlikely that those other purposes rely on the return never being negative.

Let's play safe for now and better introduce a new function that allows negative durations (e.g. ParseDurationAllowNegative or something). It will be easy to wire it to parseDuration in the templating.

WDYT?

[iamhalje]

@beorn7 actually has similar question before, i agree. what would you say now? mean what think about String method and the tests?

[beorn7]

Thanks. The code is correct, but I have a bunch of suggestions to make it more DRY and compact.

[iamhalje]

@beorn7 i really like everything, thanks, and i also improved TestParseDuration function in tests

[beorn7]

Thanks. A few more tweaks.

[iamhalje]

@beorn7 LGTY?

[beorn7]

Looks great. I'll also tag a new release so that you can use this change in prometheus/prometheus#16619