impl TypedMetric for Counter/HistogramWithExemplars

[adamchalmers]

Fixes #95, i.e. that on master, you cannot create a Histogram or Counter which has both a Family and Exemplars. Also adds an example showing how to make such a histogram/counter. This example won't compile without my fix, so it also serves as a regression test :)

Running my example gives the expected output:

# HELP latency help text goes here.
# TYPE latency histogram
latency_sum{result="success"} 0.001345422
latency_count{result="success"} 1
latency_bucket{result="success",le="1.0"} 1 # {trace_id="3a2f90c9f80b894f"} 0.001345422

If you approve this PR, would you please also consider cutting a release 0.19 for it? I think most real-world exemplars will also want to use families, so I think this feature is worth releasing soon. My team at Cloudflare is gonna use my fork with this PR for now, but I'd like to start using this crate in more projects internally, and we try to avoid relying on Github forks :)

Thanks again for making this library. I've been using the old prometheus crate since 2018, and made a few PRs here and there, so I really appreciate you modernizing it and adding full OpenMetrics compliance.

[mxinden]

Oh, my bad. Thanks for the fix. Much appreciated.

[mxinden]

Thanks for also adding an example. I wonder whether people would easier discover this as a doc example on HistogramWithExemplars or Family (potentially one linking to the other). What do you think @adamchalmers?

[adamchalmers]

Sounds good, I'll remove this example and replace it with examples on the docs for CounterWithExemplar and HistogramWithExemplars.

[mxinden]

Cross-referencing question on whether this is needed: #94 (comment)

[adamchalmers]

Ah, it's not needed! I don't know why I thought it was. Removed anyway in favour of using examples on the docs.

[mxinden]

If you approve this PR, would you please also consider cutting a release 0.19 for it? I think most real-world exemplars will also want to use families, so I think this feature is worth releasing soon. My team at Cloudflare is gonna use my fork with this PR for now, but I'd like to start using this crate in more projects internally, and we try to avoid relying on Github forks :)

👍 Yes, happy to cut a new release once this is merged, especially since that would publish protobuf support #83.

Thanks again for making this library. I've been using the old prometheus crate since 2018, and made a few PRs here and there, so I really appreciate you modernizing it and adding full OpenMetrics compliance.

Wonderful feedback. Thank you ❤️

[adamchalmers]

Hmm, given that you're shipping protobuf support, you may want to include #98 in the next release too.

[mxinden]

Thanks for the follow-ups. Two more comments. Otherwise ready to merge.

[adamchalmers]

OK, I think the indentation issue is fixed. Here's what the docs look like now.

[mxinden]

🙏