compute/metadata: unset ResponseHeaderTimeout in defaultClient
compute/metadata: unset ResponseHeaderTimeout in defaultClient

for customers who use workload identity in gke, this argument will sometimes
prevent them from getting metadata credentials under heavy load. the roundtrip
to mds usually takes longer than 2 seconds.

we probably need to do the same thing to packages in java, c++, etc.. if there
is such timeout parameter set.

Change-Id: I284a0104a0b87a53dbbddca11b1468a31692ea97
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/50350
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go