Add template var arg type checking for EventMetric.
This change adds some type checking for the variable argument
EventMetric which assures that there are no mismatches between
the definition of an EventMetric instance and the overloaded
constructor.
This is a merge from wvgerrit/28424.
BUG: 34846989
Test: Manually verified appropriate compile time errors.
Change-Id: I3e501abf76b78211f9cef19960ef11c179a0ff93
0963400d57