! 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, 4 insertions, 4 deletions

diff --git a/kamon-core/src/main/scala/kamon/trace/TraceContext.scala b/kamon-core/src/main/scala/kamon/trace/TraceContext.scala
index 307cf17a..9ce3cd4e 100644
--- a/kamon-core/src/main/scala/kamon/trace/TraceContext.scala
+++ b/kamon-core/src/main/scala/kamon/trace/TraceContext.scala
@@ -18,11 +18,11 @@ package kamon.trace
 
 import akka.actor.ActorSystem
 import kamon.Kamon
-import kamon.metrics._
+import kamon.metric._
 import java.util.concurrent.ConcurrentLinkedQueue
 import kamon.trace.TraceContextAware.DefaultTraceContextAware
 import kamon.trace.TraceContext.SegmentIdentity
-import kamon.metrics.TraceMetrics.TraceMetricRecorder
+import kamon.metric.TraceMetrics.TraceMetricRecorder
 
 trait TraceContext {
   def name: String
@@ -41,7 +41,7 @@ object TraceContext {
 }
 
 trait SegmentCompletionHandle {
-  def finish(metadata: Map[String, String])
+  def finish(metadata: Map[String, String] = Map.empty)
 }
 
 case class SegmentData(identity: MetricIdentity, duration: Long, metadata: Map[String, String])
@@ -76,7 +76,7 @@ object SegmentCompletionHandleAware {
 }
 
 class SimpleMetricCollectionContext(@volatile private var _name: String, val token: String, metadata: Map[String, String],
-                                    val system: ActorSystem) extends TraceContext {
+    val system: ActorSystem) extends TraceContext {
   @volatile private var _isOpen = true
   val levelOfDetail = OnlyMetrics
   val startMark = System.nanoTime()