perf_hooks: introduce createHistogram

[jasnell]

Adds a new perf_hooks.createHistogram() API for creating histogram
instances that allow user recording.

Makes Histogram instances cloneable via MessagePort. This allows, for
instance, an event loop delay monitor to be running on the main thread
while the histogram data can be monitored actively from a worker thread.

const h = perf_hooks.createHistogram();
h.record(1000);
console.log(h.min);
console.log(h.percentiles);

const mc = new MessageChannel();
mc.port1.onmessage = ({data}) => console.log(data.mean);
mc.port2.postMessage(h);

const e = perf_hooks.monitorEventLoopDelay();
e.enable();

const mc = new MessageChannel();
mc.port1.onmessage = ({data}) => console.log(data.mean);
mc.port2.postMessage(e);

Why would we want this? The histogram mechanism is useful in general as a part of performance monitoring tooling and since we have hdr-histogram built in and are using it for other things, it makes sense to expose. Allowing histograms to be cloned across contexts allows for more options for more efficiently monitoring when the histogram is used for performance analysis.

Signed-off-by: James M Snell jasnell@gmail.com

[mcollina]

LGTM, really nice.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/36126/

[jasnell]

Landed in 4a19cc8