Feature Request: Add a way to get the last measurement

[carl-mastrangelo]

When unit testing that counters, gauges, and other meters are correctly updated, it's verbose to get the latest numbers. I think it would be useful to provide a way to get the last measurement value on a meter. Currently, I call Meter.measure, sort the entries by timestamp, and pick the last one.

One way to fix this would be a lastMeasurement default method on the interface, that does effectively the same thing. Individual implementations that keep an ordered list of measurements can override it and provide an optimized variant. This would allow me to build an "expected" and "actual" measurement map, from Id to Double. I could compare the two maps to ensure they match in my tests.

[brharrington]

I'm not sure I follow the exact problem your having. Do you have a sample unit test showing the issue?

There are some docs on testing: https://netflix.github.io/spectator/en/latest/intro/testing/

[carl-mastrangelo]

I do, though it's internal atm. The gist of it is:

        Map<Id, Double> expectedMetrics = Map.of(
                registry.createId("some.id.1", "bucket", "bucket", "key", expectedKey1, "success", "true"), 1D,
                registry.createId("some.id.2", "bucket", "bucket", "key", expectedKey2, "success", "true"), 1D,
                registry.createId("some.id.3", "bucket", "bucket", "key", expectedKey3, "success", "true"), 1D,
                registry.createId("some.id.4", "bucket", "bucket", "count", "3", "success", "true"), 1D,
                registry.createId("some.id.5"), 2D,
                registry.createId("some.id.6", "key", expectedKey4, "status", "OK"), 1D);

        runTheTest(registry);

        Truth.assertThat(buildMap(registry)).containsExactlyEntriesIn(expectedMetrics);

Build Map looks like:

    private static Map<Id, Double> buildMap(Registry registry) {
        Map<Id, Double> actualMeasurements = new LinkedHashMap<>();
        for (Meter m : registry) {
            Measurement last = null;
            for (Measurement ms : m.measure()) {
                if (last == null || ms.timestamp() > last.timestamp()) {
                    last = ms;
                }
            }
            actualMeasurements.put(m.id(), Objects.requireNonNull(last).value());
        }
        return Collections.unmodifiableMap(actualMeasurements);
    }

The problem with this approach is that's verbose. Calling registry.count(id) is more difficult, because not all the meters are counters. Splitting each each type out risks forgetting about a certain kind of meter. By comparing the entire registry to an expected registry, I can confirm no metrics are accidentally not accounted for.

Providing a lastMeasurement() would allow me to get right of buildMap function, and make a stream based map.