observability: Improve ping logging

Created by: slimsag

https://sourcegraph.com/github.com/sourcegraph/sourcegraph@855765631d4c8236ed6f5efcc7a356ffca9a84fa/-/blob/cmd/frontend/internal/app/pkg/updatecheck/client.go#L139-220

All of the logs in this function should have a proper prefix. Right now you must manually search frontend logs for each one, which is a huge pain and made debugging https://sourcegraph.slack.com/archives/CMB6K7SMN/p1586814349064400 really hard.