Span error.message from throwable (#625)

* Config utils pairs fix

Extract key-value pairs directly from config set. Extracting topLevelKeys
and then fetching values by key will explode in cases where key is not
a valid `com.typesafe.config.impl.Path` (globs)
Fixes kamon-io/kamon-akka-http#72

* use throwable message for error.message for spans failed with a throwable

kamon-core-tests/src/test/scala/kamon/trace/TracerSpec.scala

@@ -259,6 +259,35 @@ class TracerSpec extends WordSpec with Matchers with SpanInspection.Syntax with
 
       span.operationName() shouldBe "customName"
     }
+
+    "collect exception information for failed Spans" in {
+      val byMessage = Kamon.spanBuilder("o1").start()
+        .fail("byMessage")
+      val byException = Kamon.spanBuilder("o2").start()
+        .fail(new RuntimeException("byException"))
+      val byMessageAndException = Kamon.spanBuilder("o3").start()
+        .fail("byMessageAndException", new RuntimeException("byException"))
+
+      Reconfigure.applyConfig("kamon.trace.include-error-stacktrace=false")
+      val byExceptionDisabled = Kamon.spanBuilder("o2").start()
+        .fail(new RuntimeException("byExceptionDisabled"))
+
+      byMessage.metricTags().get(plainBoolean("error")) shouldBe true
+      byMessage.spanTags().get(plain("error.message")) shouldBe "byMessage"
+      byMessage.spanTags().get(option("error.stacktrace")) shouldBe None
+
+      byException.metricTags().get(plainBoolean("error")) shouldBe true
+      byException.spanTags().get(plain("error.message")) shouldBe "byException"
+      byException.spanTags().get(option("error.stacktrace")) shouldBe defined
+
+      byMessageAndException.metricTags().get(plainBoolean("error")) shouldBe true
+      byMessageAndException.spanTags().get(plain("error.message")) shouldBe "byMessageAndException"
+      byException.spanTags().get(option("error.stacktrace")) shouldBe defined
+
+      byExceptionDisabled.metricTags().get(plainBoolean("error")) shouldBe true
+      byExceptionDisabled.spanTags().get(plain("error.message")) shouldBe "byExceptionDisabled"
+      byExceptionDisabled.spanTags().get(option("error.stacktrace")) shouldBe None
+    }
   }
 
   private def remoteSpan(samplingDecision: SamplingDecision = SamplingDecision.Sample): Span.Remote =

kamon-core/src/main/scala/kamon/trace/Span.scala

@@ -517,8 +517,12 @@ object Span {
       if(_isOpen) {
         _hasError = true
 
-        if(isSampled && includeErrorStacktrace)
-          _spanTags.add(TagKeys.ErrorStacktrace, toStackTraceString(throwable))
+        if(isSampled) {
+          _spanTags.add(TagKeys.ErrorMessage, throwable.getMessage)
+
+          if(includeErrorStacktrace)
+            _spanTags.add(TagKeys.ErrorStacktrace, toStackTraceString(throwable))
+        }
       }
       this
     }