Currently, we allow users to set the User-Agent header via the WithUserAgent client option.

There's also the "UserAgent" field in the JSON/REST generated clients (this repo) which is appended to the default user agent (googleapi.UserAgent).

This option sets the User-Agent in its entirety, which means the client doesn't send its default User-Agent. It also makes it a bit more difficult for intermediary libraries to allow their users to set the User-Agent (think about a terraform library, for instance, you could end up with three user agents: end user, terraform, api client).

We might need some way to prepend user agent strings.

Questions:

• do we actually to make a change? is our default User-Agent important?
• prepend or append?

see PR #490