Add metrics for devtools IPL

[annagrin]

Metrics to consider:

• Total time from start of the debugger to the end of first getSourceReport call
  • this is the last VM call before DevTools is ready to be used
• Elapsed time for calls on the critical path for devtools IPL:
  • getVM
  • getIsolate
  • getObject
  • getScripts
  • getSourceReport