Jetty HttpClient metrics

[jkschneider]

Performs request timing and request content size measuring with a distribution summary.

HttpClient httpClient = new HttpClient()
httpClient.getRequestListeners().add(
  JettyClientMetrics
    .builder(registry, result -> result.getRequest().getURI().getPath()) <1>
    .timingMetricName("jetty.client.requests") // optional
    .contentSizeMetricName("jetty.client.request.size") // optional
    .maxUriTags(1000) // optional
    .build()
);

<1> Make this better at replacing with path variables and stripping request params. This has to be user-provided logic, because HttpClient doesn't have a built-in path variable substitution mechanism.