Unable to know when gateway extension is scraped, no log line printed

[smarterclayton]

In GAIE v0.2, there is no log line written when the extension is scraped by a caller (/metrics on the ext_proc read), which means it's harder to debug if a scraping configuration is valid.

We should also better structure our logging to report at v1/v2 when endpoints on the extension are invoked, consistent with k/k standards for logging (https://github.com/kubernetes/kubernetes/blob/v1.4.3/docs/devel/logging.md)