RFR: metrics: add library

[Erik Joelsson]

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

Looks good, thanks for implementing this!

metrics/src/main/java/org/openjdk/skara/metrics/Counter.java line 185:

> 183:             this.name = name;
> 184:             this.label = label;
> 185:             this.value = new ConcurrentHashMap<String, DoubleAdder>();

Can leave out the types between <> here.

metrics/src/main/java/org/openjdk/skara/metrics/Gauge.java line 213:

> 211:             this.name = name;
> 212:             this.label = label;
> 213:             this.value = new ConcurrentHashMap<String, DoubleAdder>();

And here.

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Marked as reviewed by erikj (Reviewer).

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