Create measurement plan and update instrumentation spec for enwiki IRS trial.
Measurement plan https://docs.google.com/document/d/1feIislgd_6CCFMrF-1mSvNoOW4_NyDu66M_OEt5mHIE/edit?tab=t.0
Instrumentation Spec: https://docs.google.com/spreadsheets/d/1067j3sZiH8mOMJmGx7Djwu2bOnaV21EUY5Y4E4zS6YM/edit?gid=0#gid=0