Span error.message from 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/package.scala

@@ -93,9 +93,9 @@ package object kamon {
     }
 
     def pairs: Map[String, String] = {
-      topLevelKeys
-        .map(key => (key, config.getString(key)))
-        .toMap
+      config.root().entrySet().asScala.map { entry =>
+        (entry.getKey, entry.getValue.unwrapped().asInstanceOf[String])
+      }.toMap
     }
   }
 }

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
     }