When parsing logs, filter away lines that are irrelevant to the microbenchmark. (#16678)

dev/devicelab/lib/tasks/microbenchmarks.dart

diff --git a/dev/devicelab/lib/tasks/microbenchmarks.dart b/dev/devicelab/lib/tasks/microbenchmarks.dart
index 6def31a..2066112 100644
--- a/dev/devicelab/lib/tasks/microbenchmarks.dart
+++ b/dev/devicelab/lib/tasks/microbenchmarks.dart
@@ -136,7 +136,7 @@
       return;
     }
 
-    if (jsonStarted)
+    if (jsonStarted && line.contains(jsonPrefix))
       jsonBuf.writeln(line.substring(line.indexOf(jsonPrefix) + jsonPrefix.length));
   });