Add tracing event for first connection.

mojo/tools/data/benchmarks

 # This file contains a list of Mojo benchmarks. For description of the file
 # format, see `mojo_benchmark --help`.
 
 benchmarks = [
   {
     'name': 'Dart app initialization',
     'app': 'https://core.mojoapps.io/dart_traced_application.mojo',
     'duration': 10,
     'measurements': [
       'time_until/traced_application/initialized',
+      'time_until/traced_application/connected',
     ]
   },
   {
     'name': 'C++ app initialization',
     'app': 'https://core.mojoapps.io/trace_me.mojo',
     'duration': 10,
     'measurements': [
       'time_until/trace_me/initialized',
+      'time_until/trace_me/connected',
     ],
     'shell-args': [
       '--args-for=https://core.mojoapps.io/trace_me.mojo --early-tracing',
     ]
   },
 ]