eval: use rate per step for sample messages (#1732)

For the value associated with sampled events, use the rate
per step, i.e. raw count for that step interval, rather than
the rate per second. This is more intuitive for users.

atlas-eval/src/main/scala/com/netflix/atlas/eval/stream/FinalExprEval.scala

@@ -299,6 +299,10 @@ private[stream] class FinalExprEval(exprInterpreter: ExprInterpreter, enableNoDa
       }
 
       private def toTimeSeriesMessage(value: AggrDatapoint): TimeSeriesMessage = {
+        // For sampled messages, convert to rate per step, i.e. a raw count per step, rather
+        // than a rate per second.
+        val secondsPerStep = step / 1000.0
+        val ratePerStep = value.value * secondsPerStep
         val id = TaggedItem.computeId(value.tags + ("atlas.query" -> value.source)).toString
         TimeSeriesMessage(
           id,
@@ -309,7 +313,7 @@ private[stream] class FinalExprEval(exprInterpreter: ExprInterpreter, enableNoDa
           step,
           TimeSeries.toLabel(value.tags),
           value.tags,
-          ArrayData(Array(value.value)),
+          ArrayData(Array(ratePerStep)),
           None,
           value.samples
         )