cleanup(bigtable): cleanup static analyzer nit (#6067)
cleanup(bigtable): cleanup static analyzer nit (#6067)

Clang's static analyzer sees the raw call to `new` as potentially
leaking. It is not, but it is also not consistent with how we assign
proto message fields elsewhere in the code, so changed it.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp