Add new tail sampling processor policy: latency

[yvrhdn]

Description:

This adds a new policy to the tail sampling processor sampling traces based upon the duration of a span. Adding this policy allows you to sample all traces of which the duration exceeds a given threshold.

It's my first time contributing to Open-Telemetry, let me know if this fits with the other code 😊

Link to tracking Issue:

Testing:

Added unit tests similar to the other policies.

Documentation:

I have updated the README.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Koenraad Verheyden (54603c7)

[jpkrohling]

I like this, and the code looks sane. But allow me to make one suggestion: how about taking the trace duration, instead of the span? Or at least, have an option for users to make the decision based on the trace or on spans.

[yvrhdn]

But allow me to make one suggestion: how about taking the trace duration, instead of the span? Or at least, have an option for users to make the decision based on the trace or on spans.

Agree, this is a great suggestion. I'll add this tomorrow 🙂

[jpkrohling]

Alright, let me know when you need a new review. I'm out next week, though, so you might need to ask someone else to review this if you can't wait for me to return.

[yvrhdn]

Alright, I have switched the implementation to check the entire duration of a trace 👍 I think this use case will be way more common.

I wasn't sure how to calculate the total duration of a trace, I've implemented it by searching the min/max timestamps of all spans. Does this seem alright?

[jpkrohling]

Looks good! There's a whole world of possible algorithms to determine the real duration of the trace, but this simple "wall clock" approach is correct most of the time for most of the users.

I would just make sure to document this decision in the readme, so that users understand what "trace duration" means for our purposes.

[yvrhdn]

I would just make sure to document this decision in the readme, so that users understand what "trace duration" means for our purposes.

Thanks! Your suggestions have been added 🙂