Add Instant class to conveniently track elapsed and start/end times

[nicoddemus]

Throughout the codebase we often need to track elapsed times, using a performance clock, and also start/end times that given seconds since epoch (via time.time()).

Instant encapsulates both functionalities to simplify the code and ensure we are using the correct functions from _pytest.timing, which we can mock in tests for reliability.

Built on top of #13394, will rebase once it gets merged.

[RonnyPfannschmidt]

We might want to make duration itself a attribute of run result

[nicoddemus]

Not sure if I would like to expose that object as part of the public API now.

[RonnyPfannschmidt]

Suggested change

	def duration(self) -> Duration:
	def until_current(self) -> Duration:

[nicoddemus]

"current" seems a bit weird to me, how about until_now? as in "get me the duration until now"?

[RonnyPfannschmidt]

I agree

[nicoddemus]

Decided to consult a LLM, Claude says:

---

Based on the code shown, I can suggest renaming the duration() method to elapsed() which would be more natural and aligns better with the concept of measuring time that has passed since the was created. This fits well with the existing property in the Duration class and is commonly used in time measurement APIs. Instant``elapsed_s
Other natural alternatives could be:

• elapsed_time()
• time_since()
• measure()

However, elapsed() would be my top recommendation as it:

1. Is concise and clear
2. Matches common terminology in time measurement
3. Aligns with the existing property in the codebase elapsed_s
4. Is consistent with similar APIs in other languages and libraries

---

I agree, further it is the same name used by Rust's Instant (which inspired this PR):

use std::time::{Duration, Instant};
use std::thread::sleep;

fn main() {
   let now = Instant::now();

   // we sleep for 2 seconds
   sleep(Duration::new(2, 0));
   // it prints '2'
   println!("{}", now.elapsed().as_secs());
}

Agree?

[RonnyPfannschmidt]

absolutely, thanks for researching this 👍

[nicoddemus]

While at it, noticed that Duration.seconds reads much better than Duration.elapsed_s, as in instant.elapsed().seconds vs instant.elapsed().elapsed_s, so I renamed that as well. 👍

[nicoddemus]

@RonnyPfannschmidt re-requested review. 👍