HTTP client loses Timeout when using oauth

[lestrozi]

The HTTP client is created passing a Transport and a Timeout, but is later reassigned to a OAuth2Config.CreateOauth2Client.

The oauth client just returns the original client if it's disabled, but it returns a new client otherwise.
The problem is that, while this client retains the transport configuration (set on ctx), it doesn't keep other settings such as Timeout.

This can be easily confirmed by setting, on outputs.http plugin config, an URL that will not respond to SYNs (such as http://5.5.5.5) and a valid token_url, client_id and client_secret.

After fetching the token, telegraf will try to connect to the HTTP endpoint and will only timeout when the OS times out the connect syscall (usually after about ~2 minutes, depending on the TCP backoff config). The goroutine is not blocked, it keeps doing epoll, but it doesn't respect the timeout.