eval: precompute expression string (Netflix#1707)

for large group by expressions it can be expensive to
recompute the expression string for every output message.
Precompute for all messages from a given expression.

atlas-eval/src/main/scala/com/netflix/atlas/eval/model/TimeSeriesMessage.scala

@@ -124,14 +124,18 @@ object TimeSeriesMessage {
     *     Time series to use for the message.
     * @param palette
     *     If defined then include presentation metadata.
+    * @param exprStr
+    *     String view of the expression. Should match `expr`, but may be precomputed for
+    *     group by expressions with many messages.
     */
   def apply(
     expr: StyleExpr,
     context: EvalContext,
     ts: TimeSeries,
-    palette: Option[String] = None
+    palette: Option[String] = None,
+    exprStr: Option[String] = None
   ): TimeSeriesMessage = {
-    val query = expr.toString
+    val query = exprStr.getOrElse(expr.toString)
     val offset = Strings.toString(Duration.ofMillis(expr.offset))
     val outputTags = ts.tags + (TagKey.offset -> offset)
     val id = TaggedItem.computeId(outputTags + ("atlas.query" -> query)).toString

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

@@ -201,6 +201,7 @@ private[stream] class FinalExprEval(exprInterpreter: ExprInterpreter)
         // Generate the time series and diagnostic output
         val output = recipients.flatMap {
           case (styleExpr, infos) =>
+            val exprStr = styleExpr.toString
             val ids = infos.map(_.id)
             // Use an identity map for the state to ensure that multiple equivalent stateful
             // expressions, e.g. derivative(a) + derivative(a), will have isolated state.
@@ -221,7 +222,8 @@ private[stream] class FinalExprEval(exprInterpreter: ExprInterpreter)
                     styleExpr,
                     context,
                     t.withLabel(styleExpr.legend(t)),
-                    info.palette
+                    info.palette,
+                    Some(exprStr)
                   )
                   new MessageEnvelope(info.id, ts)
                 }