! 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, 55 insertions, 0 deletions

diff --git a/kamon-core/src/test/scala/kamon/metric/instrument/CounterSpec.scala b/kamon-core/src/test/scala/kamon/metric/instrument/CounterSpec.scala
new file mode 100644
index 00000000..1a93e1f6
--- /dev/null
+++ b/kamon-core/src/test/scala/kamon/metric/instrument/CounterSpec.scala
@@ -0,0 +1,55 @@
+package kamon.metric.instrument
+
+import java.nio.LongBuffer
+
+import kamon.metric.CollectionContext
+import org.scalatest.{ Matchers, WordSpec }
+
+class CounterSpec extends WordSpec with Matchers {
+
+  "a Counter" should {
+    "allow increment only operations" in new CounterFixture {
+      counter.increment()
+      counter.increment(10)
+
+      intercept[UnsupportedOperationException] {
+        counter.increment(-10)
+      }
+    }
+
+    "reset to zero when a snapshot is taken" in new CounterFixture {
+      counter.increment(100)
+      takeSnapshotFrom(counter).count should be(100)
+      takeSnapshotFrom(counter).count should be(0)
+      takeSnapshotFrom(counter).count should be(0)
+
+      counter.increment(50)
+      takeSnapshotFrom(counter).count should be(50)
+      takeSnapshotFrom(counter).count should be(0)
+    }
+
+    "produce a snapshot that can be merged with others" in new CounterFixture {
+      val counterA = Counter()
+      val counterB = Counter()
+      counterA.increment(100)
+      counterB.increment(200)
+
+      val counterASnapshot = takeSnapshotFrom(counterA)
+      val counterBSnapshot = takeSnapshotFrom(counterB)
+
+      counterASnapshot.merge(counterBSnapshot, collectionContext).count should be(300)
+      counterBSnapshot.merge(counterASnapshot, collectionContext).count should be(300)
+    }
+
+  }
+
+  trait CounterFixture {
+    val counter = Counter()
+
+    val collectionContext = new CollectionContext {
+      val buffer: LongBuffer = LongBuffer.allocate(1)
+    }
+
+    def takeSnapshotFrom(counter: Counter): Counter.Snapshot = counter.collect(collectionContext)
+  }
+}