Write correlation-id to log entries when available

[JustinBeckwith]

By writing a correlation-id into the logging metadata, users can get request-correlated logs in pantheon. See https://gist.github.com/JustinBeckwith/43dd04cfbc4551ddd8996a5b803da0b0 for details.

[landrito]

@JustinBeckwith I am having trouble seeing the gist.

[stephenplusplus]

This issue was moved to googleapis/nodejs-logging#23