! 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, 23 insertions, 20 deletions

diff --git a/kamon-core/src/main/resources/reference.conf b/kamon-core/src/main/resources/reference.conf
index d2830892..b7f5c70e 100644
--- a/kamon-core/src/main/resources/reference.conf
+++ b/kamon-core/src/main/resources/reference.conf
@@ -48,30 +48,33 @@ kamon {
     ]
 
     precision {
+      default-histogram-precision {
+        highest-trackable-value = 3600000000000
+        significant-value-digits = 2
+      }
+
+      default-min-max-counter-precision {
+        refresh-interval = 100 milliseconds
+        highest-trackable-value = 999999999
+        significant-value-digits = 2
+      }
+
+      default-gauge-precision {
+        refresh-interval = 100 milliseconds
+        highest-trackable-value = 999999999
+        significant-value-digits = 2
+      }
+
+
       actor {
-        processing-time {
-          highest-trackable-value = 3600000000000
-          significant-value-digits = 2
-        }
-        time-in-mailbox {
-          highest-trackable-value = 3600000000000
-          significant-value-digits = 2
-        }
-        mailbox-size {
-          highest-trackable-value = 999999999
-          significant-value-digits = 2
-        }
+        processing-time = ${kamon.metrics.precision.default-histogram-precision}
+        time-in-mailbox = ${kamon.metrics.precision.default-histogram-precision}
+        mailbox-size = ${kamon.metrics.precision.default-min-max-counter-precision}
       }
 
       trace {
-        elapsed-time {
-          highest-trackable-value = 3600000000000
-          significant-value-digits = 2
-        }
-        segment {
-          highest-trackable-value = 3600000000000
-          significant-value-digits = 2
-        }
+        elapsed-time = ${kamon.metrics.precision.default-histogram-precision}
+        segment = ${kamon.metrics.precision.default-histogram-precision}
       }
 
       dispatcher {