Integrated: metrics: add library

[Erik Helin]

On Tue, 11 May 2021 18:32:17 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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](https://prometheus.io/docs/instrumenting/writing_clientlibs/) and currently features the two Prometheus types [gauge](https://prometheus.io/docs/concepts/metric_types/#gauge) and [counter](https://prometheus.io/docs/concepts/metric_types/#counter). The library is designed to allow it to be extended with the [histogram](https://prometheus.io/docs/concepts/metric_types/#histogram) and [summary](https://prometheus.io/docs/concepts/metric_types/#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](https://prometheus.io/docs/introduction/overview/) 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

This pull request has now been integrated.

Changeset: 537acafc
Author:    Erik Helin <ehelin at openjdk.org>
URL:       https://git.openjdk.java.net/skara/commit/537acafcd3cfd453b909edea154fe283d8d5e628
Stats:     1234 lines in 34 files changed: 1228 ins; 1 del; 5 mod

metrics: add library

Reviewed-by: erikj

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PR: https://git.openjdk.java.net/skara/pull/1148