! all: refactor the core metric recording instruments and accomodate UserMetrics

This PR is including several changes to the kamon-core, most notably:
  - Formalize the interface for Histograms, Counters and MinMaxCounters. Making sure
    that the interfaces are as clean as possible.
  - Move away from the all Vector[Measurement] based Histogram snapshot to a new approach
    in which we use a single long to store both the index in the counts array and the
    frequency on that bucket. The leftmost 2 bytes of each long are used for storing the
    counts array index and the remaining 6 bytes are used for the actual count, and
    everything is put into a simple long array. This way only the buckets that actually
    have values will be included in the snapshot with the smallest possible memory
    footprint.
  - Introduce Gauges.
  - Reorganize the instrumentation for Akka and Scala and rewrite most of the tests
    of this components to avoid going through the subscription protocol to test.
  - Introduce trace tests and fixes on various tests.
  - Necessary changes on new relic, datadog and statsd modules to compile with the new
    codebase.

Pending:
  - Finish the upgrade of the new relic to the current model.
  - Introduce proper limit checks for histograms to ensure that we never pass the 2/6 bytes
    limits.
  - More testing, more testing, more testing.
  - Create the KamonStandalone module.

1 files changed, 70 insertions, 0 deletions

diff --git a/kamon-core/src/test/scala/kamon/metric/instrument/GaugeSpec.scala b/kamon-core/src/test/scala/kamon/metric/instrument/GaugeSpec.scala
new file mode 100644
index 00000000..b3ff3c9f
--- /dev/null
+++ b/kamon-core/src/test/scala/kamon/metric/instrument/GaugeSpec.scala
@@ -0,0 +1,70 @@
+package kamon.metric.instrument
+
+import java.util.concurrent.atomic.AtomicLong
+
+import akka.actor.ActorSystem
+import com.typesafe.config.ConfigFactory
+import kamon.metric.{ Scale, CollectionContext }
+import org.scalatest.{ Matchers, WordSpecLike }
+import scala.concurrent.duration._
+
+class GaugeSpec extends WordSpecLike with Matchers {
+  val system = ActorSystem("gauge-spec", ConfigFactory.parseString(
+    """
+      |kamon.metrics {
+      |  flush-interval = 1 hour
+      |  precision {
+      |    default-gauge-precision {
+      |      refresh-interval = 100 milliseconds
+      |      highest-trackable-value = 999999999
+      |      significant-value-digits = 2
+      |    }
+      |  }
+      |}
+    """.stripMargin))
+
+  "a Gauge" should {
+    "automatically record the current value using the configured refresh-interval" in {
+      val numberOfValuesRecorded = new AtomicLong(0)
+      val gauge = Gauge.fromDefaultConfig(system) { () ⇒ numberOfValuesRecorded.addAndGet(1) }
+
+      Thread.sleep(1.second.toMillis)
+      numberOfValuesRecorded.get() should be(10L +- 1L)
+      gauge.cleanup
+    }
+
+    "stop automatically recording after a call to cleanup" in {
+      val numberOfValuesRecorded = new AtomicLong(0)
+      val gauge = Gauge.fromDefaultConfig(system) { () ⇒ numberOfValuesRecorded.addAndGet(1) }
+
+      Thread.sleep(1.second.toMillis)
+      gauge.cleanup
+      numberOfValuesRecorded.get() should be(10L +- 1L)
+      Thread.sleep(1.second.toMillis)
+      numberOfValuesRecorded.get() should be(10L +- 1L)
+    }
+
+    "produce a Histogram snapshot including all the recorded values" in {
+      val numberOfValuesRecorded = new AtomicLong(0)
+      val gauge = Gauge.fromDefaultConfig(system) { () ⇒ numberOfValuesRecorded.addAndGet(1) }
+
+      Thread.sleep(1.second.toMillis)
+      gauge.cleanup
+      val snapshot = gauge.collect(CollectionContext.default)
+
+      snapshot.numberOfMeasurements should be(10L +- 1L)
+      snapshot.min should be(1)
+      snapshot.max should be(10L +- 1L)
+    }
+
+    "not record the current value when doing a collection" in {
+      val numberOfValuesRecorded = new AtomicLong(0)
+      val gauge = Gauge(Histogram.Precision.Normal, 10000L, Scale.Unit, 1 hour, system)(() ⇒ numberOfValuesRecorded.addAndGet(1))
+
+      val snapshot = gauge.collect(CollectionContext.default)
+
+      snapshot.numberOfMeasurements should be(0)
+      numberOfValuesRecorded.get() should be(0)
+    }
+  }
+}