! 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, 78 insertions, 0 deletions

diff --git a/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala b/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala
new file mode 100644
index 00000000..1efff2bc
--- /dev/null
+++ b/kamon-core/src/main/scala/kamon/metric/instrument/Gauge.scala
@@ -0,0 +1,78 @@
+package kamon.metric.instrument
+
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicReference
+
+import akka.actor.{ Cancellable, ActorSystem }
+import com.typesafe.config.Config
+import kamon.metric.{ CollectionContext, Scale, MetricRecorder }
+
+import scala.concurrent.duration.FiniteDuration
+
+trait Gauge extends MetricRecorder {
+  type SnapshotType = Histogram.Snapshot
+
+  def record(value: Long)
+  def record(value: Long, count: Long)
+}
+
+object Gauge {
+
+  trait CurrentValueCollector {
+    def currentValue: Long
+  }
+
+  def apply(precision: Histogram.Precision, highestTrackableValue: Long, scale: Scale, refreshInterval: FiniteDuration,
+    system: ActorSystem)(currentValueCollector: CurrentValueCollector): Gauge = {
+
+    val underlyingHistogram = Histogram(highestTrackableValue, precision, scale)
+    val gauge = new HistogramBackedGauge(underlyingHistogram, currentValueCollector)
+
+    val refreshValuesSchedule = system.scheduler.schedule(refreshInterval, refreshInterval) {
+      gauge.refreshValue()
+    }(system.dispatcher) // TODO: Move this to Kamon dispatchers
+
+    gauge.refreshValuesSchedule.set(refreshValuesSchedule)
+    gauge
+  }
+
+  def fromDefaultConfig(system: ActorSystem)(currentValueCollectorFunction: () ⇒ Long): Gauge =
+    fromDefaultConfig(system, functionZeroAsCurrentValueCollector(currentValueCollectorFunction))
+
+  def fromDefaultConfig(system: ActorSystem, currentValueCollector: CurrentValueCollector): Gauge = {
+    val config = system.settings.config.getConfig("kamon.metrics.precision.default-gauge-precision")
+    fromConfig(config, system)(currentValueCollector)
+  }
+
+  def fromConfig(config: Config, system: ActorSystem)(currentValueCollector: CurrentValueCollector): Gauge = {
+    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)
+
+    Gauge(Histogram.Precision(significantDigits), highest, Scale.Unit, refreshInterval.millis, system)(currentValueCollector)
+  }
+
+  implicit def functionZeroAsCurrentValueCollector(f: () ⇒ Long): CurrentValueCollector = new CurrentValueCollector {
+    def currentValue: Long = f.apply()
+  }
+}
+
+class HistogramBackedGauge(underlyingHistogram: Histogram, currentValueCollector: Gauge.CurrentValueCollector) extends Gauge {
+  val refreshValuesSchedule = new AtomicReference[Cancellable]()
+
+  def record(value: Long): Unit = underlyingHistogram.record(value)
+
+  def record(value: Long, count: Long): Unit = underlyingHistogram.record(value, count)
+
+  def collect(context: CollectionContext): Histogram.Snapshot = underlyingHistogram.collect(context)
+
+  def cleanup: Unit = {
+    if (refreshValuesSchedule.get() != null)
+      refreshValuesSchedule.get().cancel()
+  }
+
+  def refreshValue(): Unit = underlyingHistogram.record(currentValueCollector.currentValue)
+}
+