ci: reduce false negatives in code coverage (googleapis/google-cloud-cpp-spanner#1328)
ci: reduce false negatives in code coverage (googleapis/google-cloud-cpp-spanner#1328)

For some reason the use of `& Function` was confusing the code coverage
tool and marking lines as "not covered" when in fact they are.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp