Timer base unit is seconds

[Channyboy]

fixes #724

[jgallimore]

I'm ok with this change and generally happy to approve it. Just to make sure my understanding is correct - this only affects the values we'd get from the snapshots (the stop() method on Context above would still return in nanoseconds?), and wouldn't affect the prometheus output, or the granularity that implementations would capture the timings at?

[donbourne]

looking at behavior from MP Metrics 4.0, even with the units specified as "seconds" in the Timer annotation the values returned in the Timer's snapshot are always in nanoseconds:

    @Path("timed2")
    @Timed(name = "timed-request222",unit = "seconds")
    @GET
    public String timedRequest2() throws InterruptedException {

        int wait = new Random().nextInt(1000);
        Thread.sleep(wait);

        SortedMap<MetricID, Timer> timers = metricRegistry.getTimers();
        for (MetricID mid : timers.keySet()) {
            System.out.println(mid);
        }

        Timer t = metricRegistry.getTimer(new MetricID("com.example.demo.metric.MetricController.timed-request222"));
        long[] values = t.getSnapshot().getValues();
        String vstr = "Values:\n";
        for (long v : values) {
            vstr += v + "\n";
        }
        System.out.println(vstr);

        System.out.println("75th percentile:" + t.getSnapshot().get75thPercentile());
        System.out.println("max:" + t.getSnapshot().getMax());
        System.out.println("mean:" + t.getSnapshot().getMean());

        return "Request2 is used in statistics, check with the Metrics call.\n";
    }

results in:

MetricID{name='com.example.demo.metric.MetricController.timed-request', tags=[]}
[INFO] MetricID{name='com.example.demo.metric.MetricController.timed-request222', tags=[]}
[INFO] Values:
[INFO] 155007364
[INFO] 165321846
[INFO] 314078968
[INFO] 454136472
[INFO] 529411374
[INFO] 581802637
[INFO] 690137171
[INFO] 838741836
[INFO] 953674612
[INFO] 999309895
[INFO] 75th percentile:8.38741836E8
[INFO] max:999309895
[INFO] mean:5.692408266253115E8

so basically the units specified in the metadata of a Timer are ignored.