! 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, 116 insertions, 0 deletions

diff --git a/kamon-core/src/main/scala/kamon/metric/instrument/MinMaxCounter.scala b/kamon-core/src/main/scala/kamon/metric/instrument/MinMaxCounter.scala
new file mode 100644
index 00000000..471e7bd4
--- /dev/null
+++ b/kamon-core/src/main/scala/kamon/metric/instrument/MinMaxCounter.scala
@@ -0,0 +1,116 @@
+package kamon.metric.instrument
+
+/*
+ * =========================================================================================
+ * Copyright © 2013-2014 the kamon project <http://kamon.io/>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+ * either express or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ * =========================================================================================
+ */
+
+import java.lang.Math.abs
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicReference
+import akka.actor.{ ActorSystem, Cancellable }
+import com.typesafe.config.Config
+import jsr166e.LongMaxUpdater
+import kamon.metric.{ Scale, MetricRecorder, CollectionContext }
+import kamon.util.PaddedAtomicLong
+import scala.concurrent.duration.FiniteDuration
+
+trait MinMaxCounter extends MetricRecorder {
+  override type SnapshotType = Histogram.Snapshot
+
+  def increment(): Unit
+  def increment(times: Long): Unit
+  def decrement()
+  def decrement(times: Long)
+}
+
+object MinMaxCounter {
+
+  def apply(highestTrackableValue: Long, precision: Histogram.Precision, scale: Scale, refreshInterval: FiniteDuration,
+    system: ActorSystem): MinMaxCounter = {
+
+    val underlyingHistogram = Histogram(highestTrackableValue, precision, scale)
+    val minMaxCounter = new PaddedMinMaxCounter(underlyingHistogram)
+
+    val refreshValuesSchedule = system.scheduler.schedule(refreshInterval, refreshInterval) {
+      minMaxCounter.refreshValues()
+    }(system.dispatcher) // TODO: Move this to Kamon dispatchers
+
+    minMaxCounter.refreshValuesSchedule.set(refreshValuesSchedule)
+    minMaxCounter
+  }
+
+  def fromConfig(config: Config, system: ActorSystem): MinMaxCounter = {
+    import scala.concurrent.duration._
+
+    val highest = config.getLong("highest-trackable-value")
+    val significantDigits = config.getInt("significant-value-digits")
+    val refreshInterval = config.getDuration("refresh-interval", TimeUnit.MILLISECONDS)
+
+    apply(highest, Histogram.Precision(significantDigits), Scale.Unit, refreshInterval.millis, system)
+  }
+}
+
+class PaddedMinMaxCounter(underlyingHistogram: Histogram) extends MinMaxCounter {
+  private val min = new LongMaxUpdater
+  private val max = new LongMaxUpdater
+  private val sum = new PaddedAtomicLong
+  val refreshValuesSchedule = new AtomicReference[Cancellable]()
+
+  min.update(0L)
+  max.update(0L)
+
+  def increment(): Unit = increment(1L)
+
+  def increment(times: Long): Unit = {
+    val currentValue = sum.addAndGet(times)
+    max.update(currentValue)
+  }
+
+  def decrement(): Unit = decrement(1L)
+
+  def decrement(times: Long): Unit = {
+    val currentValue = sum.addAndGet(-times)
+    min.update(-currentValue)
+  }
+
+  def collect(context: CollectionContext): Histogram.Snapshot = {
+    refreshValues()
+    underlyingHistogram.collect(context)
+  }
+
+  def cleanup: Unit = {
+    if (refreshValuesSchedule.get() != null)
+      refreshValuesSchedule.get().cancel()
+  }
+
+  def refreshValues(): Unit = {
+    val currentValue = {
+      val value = sum.get()
+      if (value < 0) 0 else value
+    }
+
+    val currentMin = {
+      val minAbs = abs(min.maxThenReset())
+      if (minAbs <= currentValue) minAbs else 0
+    }
+
+    underlyingHistogram.record(currentValue)
+    underlyingHistogram.record(currentMin)
+    underlyingHistogram.record(max.maxThenReset())
+
+    max.update(currentValue)
+    min.update(-currentValue)
+  }
+}