! 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, 2 insertions, 2 deletions

diff --git a/kamon-core/src/main/scala/kamon/trace/TraceRecorder.scala b/kamon-core/src/main/scala/kamon/trace/TraceRecorder.scala
index 0e264cd2..efe08cdb 100644
--- a/kamon-core/src/main/scala/kamon/trace/TraceRecorder.scala
+++ b/kamon-core/src/main/scala/kamon/trace/TraceRecorder.scala
@@ -33,7 +33,7 @@ object TraceRecorder {
   def newToken = "%s-%s".format(hostnamePrefix, tokenCounter.incrementAndGet())
 
   private def newTraceContext(name: String, token: Option[String], metadata: Map[String, String],
-                              system: ActorSystem): TraceContext = {
+    system: ActorSystem): TraceContext = {
 
     // In the future this should select between implementations.
     val finalToken = token.getOrElse(newToken)
@@ -51,7 +51,7 @@ object TraceRecorder {
     traceContextStorage.set(Some(ctx))
   }
 
-  def startSegment(identity: SegmentIdentity, metadata: Map[String, String]): Option[SegmentCompletionHandle] =
+  def startSegment(identity: SegmentIdentity, metadata: Map[String, String] = Map.empty): Option[SegmentCompletionHandle] =
     currentContext.map(_.startSegment(identity, metadata))
 
   def rename(name: String): Unit = currentContext.map(_.rename(name))