Add support for all functions

[fpetkovski]

This issue tracks which functions are left to be implemented before we have complete support for all PromQL functions.
The list of available functions in PromQL is here: https://prometheus.io/docs/prometheus/latest/querying/functions/

• abs()
• absent()
• absent_over_time()
• ceil()
• changes()
• clamp()
• clamp_max()
• clamp_min()
• day_of_month()
• day_of_week()
• day_of_year()
• days_in_month()
• delta()
• deriv()
• exp()
• floor()
• histogram_count() and histogram_sum()
• histogram_fraction()
• histogram_quantile()
• holt_winters()
• hour()
• idelta()
• increase()
• irate()
• label_join()
• label_replace()
• ln()
• log2()
• log10()
• minute()
• month()
• predict_linear()
• pi()
• rate()
• resets()
• round()
• scalar()
• sgn()
• sort()
• sort_desc()
• sqrt()
• time()
• timestamp()
• vector()
• year()
• _over_time()
• Trigonometric Functions

[GiedriusS]

Why did you mark the function "rate()" as unfinished? 🤔

[fpetkovski]

Ah sorry, missed that. Marked as done now ✅

[zeus2611]

hey, i would like to start contributing to this as well. Can you guide me how i can get started.

[fpetkovski]

@zeus2611 maybe you can look into adding support for the absent function? Start by adding a test case, and work your way to making the test case pass.

[yeya24]

To support more functions, we probably need to refactor the function part a little bit. Now Variadic is not supported and I can see for functions like predict_linear and holt_winters, since the first parameter is a range vector, we don't pass the scalar parameters when executing the function.

[fpetkovski]

Since the function operator takes a slice of operands, do we need anything special for variadic functions?

For the second point, I agree that we need a better way to handle matrix selectors outside of rate et al.

[Althaf66]

I like to contribute this project. I am just newbie to this technology, so what all tools and techstack I need to learn to complete this project.

[MichaHoffmann]

Only holt-winters remaining ~ given that it is disappearing from prometheus; shall we close the issue?

[fpetkovski]

I think we still need to add support outside of subqueries.