RFR: metrics: add library

[Erik Helin]

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

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Commit messages:
 - metrics: add library

Changes: https://git.openjdk.java.net/skara/pull/1148/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1148&range=00
  Stats: 1234 lines in 34 files changed: 1228 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1148.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1148/head:pull/1148

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