metrics: add library

[edvbld]

Hi all,

please review this patch that adds the new metrics library. The metrics library can be used to collect runtime metrics, such as the number of HTTP requests or the number of bot runs. The library is designed according to the Prometheus library guide and currently features the two Prometheus types gauge and counter. The library is designed to allow it to be extended with the histogram and summary types if we find a need for them.

The Counter and Gauge types are designed to be thread-safe and should typically be stored in a private final static variable in a classes (similar to how one might store a Logger instance). A minimal Counter example looks like:

class Example {
    private static final Counter counter = Counter.name("test").register();
}

I won't go into details here on how Prometheus naming and labelling works, please see the Prometheus documentation for those concepts. The goal is that library should be fairly self-explanatory for someone with an understanding of how Prometheus works. Note though that the metrics library itself is not tied to Prometheus in any way, it only supports the collection of runtime metrics. It is however designed to make it easy to write a Prometheus exporter, something I will show in a follow-up patch.

I also added a bunch of unit tests for the new library.

Thanks,
Erik

---

Progress

• Change must not contain extraneous whitespace
• Change must be properly reviewed

Reviewers

• Erik Joelsson (@erikj79 - Reviewer) ⚠️ Review applies to 38e0ee0

Reviewing

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[mlbridge]

Webrevs

• 01: Full - Incremental (35bb1235)
• 00: Full (38e0ee00)

[erikj79]

Looks good, thanks for implementing this!

[erikj79]

Can leave out the types between <> here.

[erikj79]

And here.

[openjdk]

@edvbld This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

metrics: add library

Reviewed-by: erikj

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[edvbld]

Thanks @erikj79 for reviewing!

/integrate

[openjdk]

@edvbld Pushed as commit 537acaf.

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