`idtoken.NewClient` panics if `http.DefaultTransport` is not a `*http.Transport`

[vorporeal]

I tried to set up our application so that all outbound HTTP requests would produce OTel traces.

Specifically, I added this code to our initialization logic:

http.DefaultTransport = otelhttp.NewTransport(http.DefaultTransport)

Unfortunately, this led to panics from idtoken.NewClient(), due to this code:

httpTransport := http.DefaultTransport.(*http.Transport).Clone()
httpTransport.MaxIdleConnsPerHost = 100

Given the fact that http.DefaultTransport has type http.RoundTripper (an interface) and not *http.Transport (a concrete type), I think this code should be doing more cautious type-checking to avoid panics here.

[codyoss]

Thanks for the report, I created a PR to fix this issue. Just a note I don't believe this happens in our newer auth library https://pkg.go.dev/cloud.google.com/go/auth/idtoken

[codyoss]

The fix will be released later today in #3262